Check the alternative hierarchies and connection options while you're at it.
The difference, I suppose, is that Google Groups are run by autopilot with nobody in the cockpit, Eternal September doesn't ask the average user too many questions (it is as accepting as its name suggests), but those small server admins will ask you questions if you start posting crap, and will delete it, and ban the account.
Sorry, but you make it sound like there are many. That allow posting, and free, I think I only know one.
It's been a while since I looked. I could have sworn there were at least a handful, but either A. I might be mis-remembering or B. the landscape might have changed since I checked last.
That said, there seem to be quite few who offer cheap if not actually free access. And by "cheap" I mean, on the order of $10.00 (USD) / month or less.
I don't think 10USD/month for access to a forum is anywhere near cheap or even reasonable, no matter how big it is. I can't imagine anyone paying that for non-warez Usenet access.
In the last few years some Usenet providers are closed their doors. Aioe.org was pretty popular and known, disappeared this year. One of few ISP-s still offering Usenet access for clients, Finnish Elisa, closed their usenet server 2021.
Russian neva.ru was sadly closed (probably forced by goverment) 2021 (I group it with free servers, because it had read-only access open for everyone and some point allowed posting). Albasani.net disappeared 2020.
Some free servers are still around, but don't offer public accounts.
Few places give account, if you ask.
Eternal September is strongest-biggest still running and free service. Mixmin allows anonymous posting, but sometimes restricts it to fight with abuse.
Google still relays groups to Usenet, though they have their own thing going and some servers are blocking posts from google.
Sorry, but you make it sound like there are many. That allow posting, and free, I think I only know one.