I have been having this exact same issue lately.

My solution was to use an sms app from f-droid called quik-sms. https://github.com/quik-sms/quik https://f-droid.org/repository/browse/?fdid=dev.octoshrimpy....

You can create a filters based on message content, to block the message. It seems to work.

I just have the block message filters working on messages only from phone numbers that are not in my contact list, and put filters like "STOP2END", "donation", etc.

Unfortunately there's no blocked/"spam" folder. If there was a "spam" folder for sms apps based on message content filters, that would probably be an almost 100% complete solution.