ErgoMechKeyboards

I haven’t built my Skelytl yet, but just completed my first build (Corne), and was in a similar situation, so hopefully my experience helps.

1. I went wireless and ZMK was fairly easy to work with, after using other’s configs for ideas. The initial ZMK build was quite straightforward. I got the nice!displays, so I could see layers. On my Moonlander, I use colors to easily identify what layer I am on. From what I have read, if you want lights, stick with wired, due to the energy draw. I have not yet connected to multiple devices, but plan on using that functionality.
2. I hadn’t soldered in many years. Searched on YouTube for some examples of soldering PCBs and Joe Scotto has a nice build tutorial for the Corne. Take your time, get a solder sucker to fix mistakes, and perhaps a magnifying glass (I used my iPhone with zoom) to check your work.
3. On my Corne, I am using some chocs, but on my Moonlander I tried the Bobas (borrowed a couple from a friend - they were nice ) but am using Tealios v2 (linears - very smooth and have a nice sound) which I have been really happy with. For a different build, just ordered some Drop + Invyr Holy Pandas, to try something different.

Good luck - while occasionally challenging, I really had fun with it and glad I did it.

Wireless? I thought about going wireless and ordering a pair of nice!nano controllers. I assume it will be a bit easier to assemble (less components) and I can switch between devices easily, but I’ve heard it’s a bit harder to troubleshoot. Is this a good idea or should I go wired for a first build? How do I go about connecting the battery and which batteries should I look into?

I don't think it's meaningfully easier to build a wireless board, since you have to deal with batteries and (preferably) power switches. It's also more expensive to get two nice!nanos and batteries (although there are some other wireless controllers available now, some of which are probably cheaper).

ZMK, the firmware used for most wireless builds, is pretty different in its workflow from QMK. It's great, but there's not as much documentation and it is indeed harder to troubleshoot as the error messages on builds are less informative.

Re: how to connect batteries, it depends, but if you're doing a fully handwired build, you you can just wire the battery directly to the controller. A power switch between the controller and the battery will help a ton if you ever need to transport the board, since there's no soft lockout in ZMK.

Re: sourcing a battery, there's a type of very small lipo battery that is often sold together with the nice!nano by vendors. Those work fine if you don't need any power-hungry features like lighting or a screen. You can get bigger equivalents of those batteries in a huge range of sizes as well, if you want. You can source them from lots of places.

Compatible switches? I’ve looked into tactile switches and loved the sound (or lack of) Gazzew Boba U4 Black Silent. Would these be compatible and a good alternative to standard cherry mx switches?

Shouldn't be a compatibility issue, any MX switch should work. One thing to watch out for though would be, for 3d printed builds, using switches that were out of spec in terms of housing size. Some switches are a little bigger or smaller than others.

One major downside to using nice!nanos for a Bluetooth setup is that the keyboard will likely experience more latency and unreliability than if you used wires. I built a redox using these several years ago, and I would regularly (but not consistently) experience more latency from one half than the other, which led to me getting the letters in words out of order. I haven't heard others complain of this, but I ended up not using the keyboard because it was so frustrating.