Forgot to say: the retention policy on the trash isn't an option either, I need to keep the trash for a few years, in case I need to go back to find something, and I most definitely don't want the spam garbage messing it up.

I would suggest that you do not keep a long history of email in Trash. Trash folders are designed to be emptied automatically, so you are in danger of accidentally losing all of it if any normal trash cleanup process runs either on your computer or at your email provider end. There is an Archive system in TB you could use to save older emails long term, or you might move emails to a specific folder other than trash if you want to keep them. That way you can set Thunderbird to Empty Trash on exit, and have all the .ru messages filtered to trash. That does what you need - filter Delete moves them to trash, then TB can empty trash every time you close TB.

Since the easiest way to get rid of messages and yet keep them around is the delete button, I reject your suggestion.

No folders should be "designed as emptied regularly" if the user wants to keep things in it. It would be a very closed-mind approach to software design. Lucky it's complete rubbish; not even Microsoft have a mandatory or default rule to clean-up trash regularly.

I've used this approach for 20 years now, thank you, it works.

What's missing is a proper perma-delete by rule.