That’s illegal, unfortunately. Only qccredited investors can invest into private companies. There should be a lower limit on that rule, say $100 or something, so naive investors could invest play sums in potentially shady stuff. But there isn’t, so you can’t.
I think it theoretically can’t be done. The protocol allows anyone to query for posts, and it has to work that way unless you want to move ActivityPub federation to an invite-only system. On most servers I think browsing while being logged out is sufficient, just like on pre-Musk Twitter.
Mastodon also doesn’t let you prevent people from reading your posts.
Mounting solar panels on roofs - like all roof work - is dangerous.
If you use something often you learn to handle the bugs and “it just works”. If you use a product rarely then it’s not gonna work as well