Reliability will come down less to how you solder and more with how your wires matrix is built and supported. Even a bad solder joint is often okay, it's the wires yanking and bouncing that dooms the joint, good or bad. If it's a hard mount joint (switch to switch in a plate), support it so it can't move, if it's flexible (switch to controller) let it flex but not flop, this will reduce the joint stress and even a bad solder joint will function for years so long as it functions, which is easy.
RGB with QMK is EASY. If you get addressable rgb (all it supports I think), it's 3 wires in series (pos, neg and signal) and a couple lines of code. Pos and neg run from the USB, and the signal wire goes to the controller, then you just designate that pin RGB. It was the easiest part of my entire board. But, and there's a mighty big but here, trying to get that rgb led under your switch in a hand wired board, forget it. You can do underglow, accent lights, whatever, but don't try getting them under your switches for backlights, the backing pcb on them is too large. It might work without hot swaps like I used but it would be a massive nightmare regardless. Even if you can get them under the switch it adds thickness, something you don't want at your front edge.
Rework and troubleshooting - this depends massively on how you wire it. I used the diodes for one half of the matrix (common) and then solid core cat5 for the other half. This gave me light, firm wires, not floppy wires, this made it easy to build assembly line style, avoiding mistakes and making it easy to spot mistakes as I went along. This will also help keep things tied together and as I said before, not flopping around breaking joints. From here I used soft core/flexible wire to jumper from the matrix to the controller, this can flex all it wants and allows some give between the hard wired matrix and the remote mounted controller.
If you want to use hot swap without a PCB expect troubles.
It can be done but why do that to yourself? It's asking for trouble. Either use a 3d printed tray like I did or just hard wire the switches, you're creating so much work to avoid soldering a switch, when it's super easy to just solder or desolder a hand wired switch anyhow. Meanwhile the hot swaps add complexity, hassle and more points of failure, especially if they are not properly supported with some sort of undertray. If you want hot swap either make sure it's supported or build a pcb with them. I get it, you can't how swap if you solder them, but you can't really have it both ways. I'm sure people try, my advice is don't. Hot swap is already a hack, this just makes it incredibly worse.
A better option is..
If you made one, you can make another. Sure making a new plate, wiring a matrix and soldering to my controller harness takes time, but built it, it's built. I can print a new "pcb" and plate for a few bucks, then along with another $10 controller I'd have an entire second plate of switches for about the same as a set of hot swaps. This may be beyond your ability or you may not want 3d printed but order extra plates so you can do the same. Don't swap switches, assemble plates of switches and swap the case instead. Once done, this is faster to switch than hot swap. being 3d printed I can also just print another case as well for a couple bucks. I did hot swap on mine for an engineering challenge but it really just doesn't make sense when you really think about it. If I were to do it again I'd go out of my way to make it just as fast and simple as possible to assemble a fresh matrix rather than hassle with hot swaps because once it's done you never mess with them again. Don't get me wrong, it's neat, but just not as practical as you think.
Hand wired delicate? I'm pretty sure I could toss mine off the roof without damage to anything but plastic (case and/or switches) if it landed on concrete.
Shorting the matrix is how you activate keys. Other than shorting 5v into your matrix you can't really hurt it by shorting anything. Odds of you needing a multimeter is slim to none as all hot wiring is in the controller. The matrix is just directing signals, crossing any of them just sends the wrong signal, A becomes H, J becomes D, etc..
Maintenance? I haven't had it long but mines fully enclosed with all wires supported in very controlled tracks, I don't expect any more maintenance to be necessary than my other boards.
Other advice,
The only trouble I had was incorrect numbers on rows and columns, the firmware looks at it as if you're looking down at the keyboard but I was looking at it from underneath so everything was mirrored left to right. Oops. While it sounds simple to just mirror the wires, they don't mirror. The labeling is not alphabetical or numerical so you can't just swap A with F, B with E, C with D and so on, nor are they in a a common order.
Don't worry about the matrix looking as good as pics of other people's matrixes, the busier it gets the more it hides. Not that it mattered electrically but I was trying to make mine look nice and straight and as I went I noticed it really didn't show as I added more and more to it. It will look neat and tidy with almost zero effort by the time you finish so don't worry about that little kink or bend.
If 3d printing try and find a printer large enough to do it in one piece and use PETG, PETG has better sound characteristics than the alternatives and a softer feel. Unfortunately PETG doesn't glue well so if you can't get a big enough printer you would be better using something else. You can print ABS and Polycarbonate, but not at this scale so they will need to be broken up and glued together. While PLA prints easiest at this scale avoid it if you can, it has a harsh sound and feel.
No matter how cool you think having an LCD on your keyboard will be, retaining your sanity from getting such a completely useless addition to your keyboard is far cooler. I don't think I worked so hard for something I cared so little about in my life. While I did get the LCD to partially work I spent more time on that than the rest of the board combined and it completely burned me out on the keyboard so much that I haven't used it since. I threw it into a box of keyboard stuff in a closet and haven't even looked at it lately. The tutorials on getting it to work, especially how I wanted are just not there yet. Admittedly I threw the kitchen sink at it, but that was kind of the idea for the board.
Lastly, don't over think it, get going on it.
The more you sit around planning, the longer it is before you get started. This probably won't be the last one you build so allow yourself to be free to make mistakes and even hate it when you're done. Once done you now know you can do it again faster and better, but until you get started you're just spinning your wheels.
