TKL is definitely more popular than 75% and if you want custom caps I would be very wary of 75% (and 65%) as there is no "standard" compared to other layouts.
For $400 you will be looking at a kit or modding an existing board, especially since probably no one sells a board with those switches. Note: So long as the board is MX compatible they should fit.
As mentioned, most kits come from China, especially in that range and they will be difficult right now. Beware kit prices, they can be quite deceptive, sure the kit is $200, but by the time you add switches, caps, better stabs, maybe some springs or o-rings that kit can quickly double in price.
If you really want to dive in though, modding an existing board isn't a bad thing and should actually be higher on people's list, especially if you plan on using it daily. Parts are easy to get and they tend to be fully fleshed out, true, most are not programmable, but some are or can be made to be for pretty cheap, though I would recommend you think about why you need it and if it can just as easily be done with software, some may laugh, but software can be extremely powerful, seeing as how it can detect what app you are using, unlike the keyboard. Meaning you can automatically disable winkey when a game starts.
If you want to build, while it won't be flashy, there are several Cooler Master and Filcos you can get replacement controllers for as well as different cases. With some work they can even me made to have a universal plate allowing you to open and swap switch internals too. Best of all, if you mess it up, replacement parts are easy to scavenge. You won't feel as bad burning up a Cooler Master PCB as you would a $400 that will take months to source a new PCB for. Best of all, you end up with a VERY reliable board, that is super strong and several options, but you can also get one now, not in 6-9 months and you might learn some new skills. For what you have to spend, you can build one heck of a hefty, reliable keyboard that will last forever. Take a look at my signature for what all I did to a Filco, the only stock remaining part is the bare pcb.
Don't forget to also look at used if you plan to use it and don't be scared of a minor blemish. Sure, everyone wants pristine, but how long will it stay that way under hard use? Not very, so why pay a bunch more for something just to look "perfect" for a month or two, that goes double for a blemish on the rear or bottom. I get it if it's meant to ride a shelf, but if you use it, it's going to get some wear and tear. Too bad you aren't closer I have a YMD75 that's never even had switches installed, nice board, it's just not for me.
