There are still an ungodly amount of mods for literally anything. All of the Vanilla Expanded mods are very high quality, plenty of older mods are still either updated or have been remade, and there are plenty that integrate with the new systems added by the DLC.

Depends how far back you go. Obvs, not all mods are keeping up as the game updates, but if anything, I find that the modding community has only every grown and gotten better with time.

I think that mod versioning was introduced in 1.1 (so about when Royalty released), which made it a lot easier to keep track of what works and what doesn't, so you might have issues with mods from before then, but honestly there'll almost certainly be a replacement if something was popular and was discontinued.

sos2 is on an experimental git branch. It has a few positive changes but also some negative ones. For example it simplifies the way shields and heat works. On the other hand, ship salvaging has to be done by hand and is no longer automatic which REALLY makes the game drag on and on and makes ship combat really really shitty since it takes 1 year just to harvest a ship and profit off of a fight. Also the ship combat has been rebalanced to make it easier for the enemy in ways I don't agree with. I was able to hack most of the stuff I don't like with dnspy but I haven't figured out how to reverse the salvaging yet.

Android Tiers, another important mod doesn't work as well anymore but there's a VE android mod now. I still use Android Tiers just because I don't like mechtech crap and I like the feel of using "alternate" technology. Beyond that, any old 1.3 or 1.2 mod that doesn't work anymore has been replaced with something better anyway.