Getting back into rom hacking, out of the loop

I've recently realised I've been out of the loop for quite a few years regarding the state of main series GBA romhacking. Last I checked, there weren't really such things as rom bases full of new content going around and our capabilities were relatively limited if you weren't highly skilled at programming, especially when scripting was involved (that was all illegible hexadecimal-heavy code back then) - heck, we could hardly expand the Pokedex without heavy glitchiness.

I did some elementary hacking with Ruby several years ago, but I only ever imported sprites, repurposed/edited rooms and edited Pokemon spawn lists (with my peak achievement being adding an overworld encounter with Deoxys which involved a bit of copy-paste scripting).

From what I gather, the hacking scene has kind of exploded in the meantime and I'd love to get (re)acquainted with the technology we have (decomps, rombases, new tools...). I've heard things are much more accessible now, but there's been so much development that I don't know where to start understanding what is what. Any pointers and recommendations for what to check out in terms of tools and what can be done with them, and for which games (like, I hear Emerald is now better understood than Ruby, go figure)?

Yeah, Emerald is probably the best rom disassembly to start with if you like Gen 3 and have a modern computer, although Pokefirered isn't far behind. I'd head over to the disassembly tutorials, or even just check out one of the disassemblies themselves as they usually have a readme with instructions on how to install and then start . If you know even a little bit of programming, it's often pretty simple to just start changing things. Simple things like image swap edits or renaming pokemon and changing their movesets don't even really require coding knowledge.

If you have an ancient thing that barely runs and belongs in a crypt, Gen 1 is pretty great; there's a full color hack for Red now, you can add day/night, items, in yellow you can make more pokemon than pikachu follow and pika evolve, etc. I am of course rather biased.

Gen II does everything Gen I does except Follow Me at the moment last I checked (although someone had a very rough version so maybe that was updated), and I know there was progress on making a full modern special att / defense move split in it but I never checked to see if it was completed. The physical / special move split has been done in Gen I and II.