Hello
While I can help you, what you are asking is very broad. Plate material, Mounting style, Switch choice and keycap profiles are all preference (plus aesthetics, typing angle and sound) But I can lead you to the right direction. (also as I'm writing this I realised that is too long for it's on good but hope this helps)
First you should go to a meetup if there is one in your area, that is the only way to try many things to see your preferences for cheap and without committing to something that you may not like (trust me, you don't want to risk buying something and just not like it, especially in a hobby that can get quite expensive) this is applicable to mounting style, switch preferences and keycap profile (not really material, at least in terms of sound, it depends but the melbourne meetup that I went had like 200 people and you couldn't really hear much)
Try everything, while it might not be what you are looking for, it can help you see what you like (If you really like the unikorn but not anything else, you could probably assume a 65% o ring mount is great for you) same with switches, maybe you like the linears in the tkl but not the tactiles in the 65. You might also like other switches than mx far more, I found out that I really liked buckling springs from typing on one. So you might as well.
Also don't really listen to sound tests as well to gauge whether a keyboard is good sounding, not only are most sound tests disorting the sound (most don't use dual stereo, instrumental microphones which is the best for recording keyboard sounds) making it inaccurate, on top of there being so many variables that determine sound that is basically impossible to completely replicate the sound (but you can get close). I guess it's good for determine what sound profile you like but just know that most tests aren't very accurate.
If you can't go to a meetup for whatever reason, then you should buy a hotswap budget board (I guess a bakeneko65 would suit you) and experiment a lot. Different switches, spring weights, plate materials, keycaps which allows you to know your preferences a bit more. It's more limited to mounting styles (a better board would be something that can be done with multiple mounting styles. Tgr 910 me for example can be built with both top mount and o ring mount, but something budget is needed), putting stabs here as well. I'd reccomend cherry clip ins and then clipping the bottoms and lubing with XHT-BDZ for the wires and 205g0 for the housing. There are guides if you need a visual explanation.
If you need simple recommendations for switches. Gateron yellows are very cheap and very nice for their price, for tactiles there are tons of options, but gateron browns if you are looking for something light or any of the wuque tactiles for something more heavy (these are also a lot cheaper) For springs I really like tx springs, just get some springs in different weights (I would reccomend 60g medium as a base point and would go up and down depending on how heavy or how light you feel they are, and how easy or hard it is to press and then use the one you like the most) Also learning to lube is great as it does make the switches smoother, there are tons of guides out there, just don't overlube as that makes a switch feel really bad.
For pure quality when it comes to keycaps, for cherry in terms of abs older gmk/cherry doubleshots are pretty nice (new gmk is alright but qc isn't the best and texture is off). Keykobo is a new maker that is very nice and cheaper than gmk and dcs keycaps are good but expensive. Pbt I would reccomend CRP or jc studio/leekbrothers if you want something much much cheaper. However they may not fit what you are looking for aesthetically since they only do beige.
In terms of case materials, I like aluminium cases with brass (or a heavier metal) weight/s in general because of the heft and sound that comes from it, but you should look at pc/plastic cases as well to see if you like them.
Once you know what you want then buy something higher end that aligns to your preferences, just ask people whether they think it's a good project or not. I usually look at a recap sheet that one of my favourite keyboard youtubers/streamer blacksimon, who is very experienced at keyboards does at
blacksimon.tv/recap helps keep track on a lot of the new projects coming out. You should get different opinions like I do but generally I think it's a good base point to go off of in order to see if something is good.
In terms of reviews about these customs a lot of youtubers are quite biased because they get sent free review units and sometimes sponsors (blacksimon is a bit different because he runs his channel at non-profit), this applies to pretty much all of them, even people who I do trust their opinion on. Disregard any opinions on people who are sponsored by keyboard companies and take people who have affiliate links with a big grain of salt as they do have something to gain from purchases.
For 65s the highest end, are the kohaku by singakbd and the tgr 910 me by tgr and that's what I would reccomend. They are really nice but really expensive (especially the kohaku). Keep in mind these boards do get flipped on the aftermarket so only buy these they really appeal to you and fit your preferences and not at an incredibly flipped price, I do see some alright deals and prices for boards are going lower but unless you know someone who is willing to do it, I seriously doubt you could find it retail. Of course this the highest end of the highest end and you may not like them, if your budget can't accommodate it or doesn't fit your preferences then buy something lower priced, there are tons of great value deals out there.
For any high end board (the one you are committing to), use the switches, plate and keycap material/profiles you like obviously. For sound the boards I mentioned really work with cherry switches, but they do require a lot more work to make smoother as they do need to be 'broken in' a bit and then lubed and spring swapped (spring swap even if you like the weight, this applies to ALL SWITCHES as well) and you do need to cherry pick a bit, but you do get nice sounding and smooth switches at the end. But use what you like and adjust the sound accordingly.
Also for a high end board, I'd recommend soldering, it makes the sound better and hotswap to me, while convenient in terms of swapping switches it can get annoying with sockets potentially falling out and switches being pulled out when you just want keycaps. Blacksimon has the guide that I use which is the best resources for keyboards at least. For a starting temp he recommends 325 which I recommend doing about 330-335 as it helps melt the solder in the way that he does it much easier (minimises error) -
For the pcb just make sure it has proper esd protection, the layout support you like and that it's remapable with qmk/via which is probably in most pcbs nowadays (also download vial if you are going to use via, I enjoy it more since it can be used offline and it's quicker, plus I don't have to use google as it doesn't support firefox last time I checked)
I like braided cables with connecters just in case I need to use a different port (like usb-mini or something) there are quality makers, I have one from space cables that looks nice just vet them as there are a weird amount of cable scammers.
If you want my preferences as a base point, and If had to build a high end custom heres what I would do, I'll do a tkl since I don't have one atm and I do like the layout.
Board - Matrix 1.2og (tried one at a meetup and it felt really nice, it's top mount, would get a wkl version)
Plate - Aluminum (Quite like how the alu plate felt with this one)
Switches - Cherry Blacks (Broken in, Lubed with ptfe powder cause that's what I really like atm, 62g medium tx springs with a oil covering them (get victorinox or a similar oil it's cheap and does the job, springs don't need to be that precise) and all the switches are filmed cause cherry switches do need films, but if I was using milky yellows I would only film the ones that the housing tolerances are more loose)
Stabs - clipped cherry clip ins - XHT-BDZ wire, 205g0 housing
Keycaps - probably a high end cherry abs set like gmk or keykobo, I'll just say a beige set, maybe I'll do pbt if the sound is a bit sharp/high pitched.
I know this is really long and a massive info dump but hope this helps anyway, good luck with finding your perfect keyboard, feel free to ask questions if you need them, I would be happy to write and even longer response lol.
