Then create a simplex group and invite everybody there.

Sometimes, but that's it. Authorities and signal itself can only say "this number has account with us since $signupDate and used it last on $usageDate". Signal can't say "we know $number is talking to $otherNumber" nor can they say "$number is in $group talking to $users".

Signal is a walled garden?

Feel free to create the chat rooms and share them as a post :)

Which ones do you suggest?

Travel across the border where you can? Ask a friend abroad to send you a SIM card? Maybe even find a service to buy a SIM from?

Go ahead and create the group then make a post. Nobody's holding you back my friend.

Same in a few other European countries. I'm doing it right now with a few SIMs. You can also go on holiday to another country, get a few temporary SIMs there for a few quid and fly back home with the "contraband". Really not hard.

Feel free to make a post about it. Very few will see it here otherwise :)

What are the "popular" alternatives? Telegram stores everything, WhatsApp doesn't allow usernames, Matrix requires IPs too...

Make it and share it. I don't have simplex.

Signal doesn't collect IPs and therefore can't even hand them out. It's been requested in 2021. Here's a list of requests from authorities they were allowed to publish. I've looked through 3 of the most recent and nowhere do they reveal IPs.