Help needed with my first build!

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.

I'm almost done building a from-scratch keyboard with a pair of nice!nanos. So far is been a pretty good experience. I ran into some issues setting up my firmware for a brand new keyboard layout (which shouldn't be an issue if you're just making a config for an existing keyboard), and the ZMK/nice!nano discords were very helpful and got me up and running.

When wiring the battery, I'd say get the biggest single cell lipo that will fit in your case. But even a 100mAh battery will get you pretty far off you don't have LEDs. You just connect the B+ and B- pins on the board to your battery. If you want to make the battery last longer, stick a switch inline with the battery to be able to completely turn it off (rather than it just going into deep sleep).

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.

I’ve only made one hand wired board, a 50 key flat, one piece ortho, but I learned some good lessons.

First, the mental hurdle for me with soldering was that you’re not JUST melting the solder, nor are you trying to get the components’ leads hot enough to melt the solder, which might fry them anyway. Instead, heating the wires or leads makes them attract solder when you melt it with the iron. Once I watched enough videos to make that jump and to see how it’s done, I went from hopeless to mediocre, and mediocre is good enough to make a keyboard. Maybe I’m just thick, but my general takeaway is that anyone can learn this.

Second, most MCUs have the port on them already. Unless you were talking about having the two halves not connect to each other, then there’s no component or labor savings in going wireless. I also was able to use KMK on a pi pico for my MCU, which was a godsend for me as someone who doesn’t code in any meaningful way.

Finally, If there’s any way your plate (maybe more of a “bowl” for a manuform, lol) and case can physically support it, consider soldering to Kailh hot swap sockets rather than switches. My little “Planck+3” board is not true hot swap, but it should be no-solder swap if I want to change the switches.