If those groups you're subscribed to are reasonably sized - up to a few hundred subscribers - you can easily run an XMPP server as a backup for whatever commercial messaging service you're using. Use Conversations on Android, Gajim or Dino-im on Linux and whatever alternatives to those are available on other systems which support OMEMO encryption (which is end to end, taking the server won't help the TLA to read your conversations) and you're set for the eventual demise of Telegram once it does its IPO and has to start showing a positive cash flow.

Going to Meta-Facebook-owned Whatsapp is not a real alternative if you're in any way concerned about privacy or autonomy. Hosting an XMPP server is dead easy, apt install prosody and a few configuration steps and you're done. It hardly takes any resources and is nearly maintenance-free (for me 'totally maintenance-free', I have never had to fiddle with it since installing it years ago even though it has been updated as part of the normal update cycle).

Not my groups, owned by non-technical users, so I doubt they will accept that suggestions. Everybody uses Whatsapp here, so it's the path of least resistance.

Anyway, I don't believe this is going to last, unless Telegram itself decides to leave Spain. They've ignored a court request for information, not even a takedown request and they're now learning that they can't do that.

People is resorting to the app's built-in proxies, but if Telegram doesn't answer, I'm afraid that hole will be fixed too.

This is not what the judge says. It says that it was the Virgin Islands the ones that did not provide information: https://twitter.com/davidmaeztu/status/1771583859672441174

That's right, that information was known later.