I'm interested in learning one or more new abstract games that are deep, and, have serious expertise to draw from, at least in the form of game records from strong players.  Ideally there should be strong AI in a user-friendly format that communicates its move evaluations; but that may be asking too much.

Can you recommend anything?  I already play Hex and Go (and I've heard of Chess, but am looking for less famous games).

Hex and Havannah (although Hvh does not have strong bots).

Twixt PP is similar to Hex and not very well known.

We have some excellent players and a robust collection of commentated games (http://twixt-commentator.duckdns.org/) and top-tier players often respond to questions you post about your own games or others.

There's also a very strong bot that is available to play by invitation and makes a move every 5-10 minutes or so. No evaluations available, although there are some commentated games where the bot's author has posted its analysis in retrospect.

I hear it's sharper and more tactical than Hex; I'm not super familiar with Hex or Havannah so I can't say. It's definitely a lot more tactical than Go — similar sense of whole-board thinking, tenuki, influence, etc, but the opening is over in 5-ish moves instead of 30.

As a go player for a long time (4k peak on ogs) I was enamored with Twixt immediately and I'm really glad to have found this outlet for it.

TZAAR, I don't know about its AI, but its strategy is totally different, plus it is not a connection or territories game.

Highest championship participation numbers could imply strength in the top division

At 112 StreetSoccer is the highest, in 2005 it had 412 players which could be a record

Tried the game myself, but played like my local team, which has 85% of been relegated (according to 5 30 8 project)

If you enjoy Chess, you will probably enjoy Amazons. I'm addicted to it, and there are plenty of strong players on this website, in addition to strong AIs.

i'm just going to bring up Dots and Boxes since littlegolem used to have the worldwide strongest players play here, so the game records are there. There is a couple papers on the math behind the game and I wrote a guide for practical play myself as well.

That being said, while it's my favorite abstract game, the playerbase nowadays is lackluster and frankly it will be difficult to find any non-retired players that you won't completely crush if you are talented at the game and reach a certain level of strength. If you or anyone else still wants to learn the game though, I'm always happy to teach people the game if they are interested in that (preferably communicating on discord).

Can you share your guide for Dots and Boxes?

Yeah, sure: https://www.docdroid.net/2Ch8voZ/dots-boxes-5x5-pdf

Also, something I forgot to mention that may be relevant to wanting to learn the game, is that the 5x5 version of the games has been computationally solved (by our very own William Fraser!)

Oh, that I knew.

Even with IGG and the iOS app, I can't imagine there's another Catchup-playing community out there - certainly not at this level.

Connect6 is likely the same story, but I'm not sure what kind of groups are still active out of Taiwan, and there's a language barrier that makes it hard (for me) to know.