Thanks for the suggestion, but I honestly don't think that's a great idea - most of our members have stumbled upon this forum through search engines and later joined it, myself included.
We did attempt to gate off the forum once in the past, and it didn't turn out very well.
Anonymous visitors are to be expected when hosting any website, it's not really a problem in itself.
I'll add though, that the web in its entirety is being scraped and crawled at a much higher rate after the LLM/"AI" boom, so I suspect some of our incoming traffic are bots who will never directly interact with the site beyond observation.