congrats on your novel progress! 
I feel you on that. WK is a great boost for me on illness days when my head will play along. The community is so friendly!
I’ve done those and they are awesome! I don’t anymore because it’s not currently how my brain enjoys the forum, but the very first one I did, the Spring 2023 challenge - I totally credit that with giving me a huge jump forward in my Japanese, helping me meet other folks here, and establishing a Japanese reading habit that remains to this day. Similarly, I did the Listen every day challenge once and felt like I got a lot out of it.
Other things on the forums - well, you’re doing the study log… I also enjoy following and commenting on other people’s study logs (haha, like this), lots of activity around book clubs (joining book clubs, leading book clubs, making up informal book clubs to lead, nominating books to book clubs), participating in taiyousea’s equinox readathons (next one coming up imminently!), participating in themed threads like the classical music thread, the travel logs are so cool, and occasionally looking at new posts I’m not following. I have to admit, at some point, when I was following so much that it takes a while to see all updates, I don’t actually look through the general feed or new posts. I only do that occasionally. In the beginning, it was a fun way to find new stuff, but I’m a bit maxed out atm so I don’t even try to keep up with what’s new. But it means sometimes I’ll totally miss someone’s log that I interact with a lot. So I review it occasionally, but I ruthlessly mute anything that turns into a dumpster fire (mercifully very rare here).