I think I can alleviate some of your worries. The instance post only shows the top 20 instances for a software (Lemmy has 287 active, Piefed: 59, Mbin: 16).
My Fediverse Growth Post is a better indicator for the Threadiverse's health. From there you can see the Threadiverse is only down 2.1% from a year ago (-847 MAU). From what I've seen over the last >6 months, is that the Threadiverse, and most of the fediverse, grow in short spurts and then slowly declines until the next spurt.
So, unfortunately, we aren't having continuous positive growth, but I'd say we are stable. I think stable is fine as all the fediverse developers are still adding functionality/interconnected-ness ('this isn't our final form').
For HYAU (Half-Year Active Users), I think it's important to remember that lots of times people move instances, so their old account will eventually stop being counted as active (I personally have 2 old accounts). From the Fediverse post, you can see there were 42889 MAU 6-months ago and 42276 MAU 3-months ago (-613 MAU). So, even though you may see >1000 HYAU drop off in a given week, the total number of active users over that next 3 months stayed relatively the same (-1.4%, not -28% (-1000 MAU x 12 weeks)).
I think the most important information in the instance post is MAU/HYAU (%) ("Stickiness"), which indicates how well an instance holds onto its users (they keep coming back). I think >60% should be the goal of every instance (67% is currently the peak for non-small instances (>30 MAU)).
It might be worth a study to figure out why there is such a discrepancy across instances. For example, is it correlated to UI issues/customizability, reliability/up-time/longevity, admin/mod behavior, instance rules, federation/de-federation, inter-instance activity (communities/posts/comments), etc.
Regarding politics, I can't really say what an average or new user experiences. I've curated/blocked massive amounts of political content. It might be worth having an instance that initially auto-blocks all politics communities for a new user. I don't recall exactly, but I think Tesseract? was like that, but the dev/admin was sketchy about what they were doing and then melted down when called out for hiding things. So, it seems the idea had some popularity, but needs/ed a better implementation.