One thing I've been able to tell friends is to just replace their default SMS app with Signal, since they're not particularly attached to that app anyway. Then, at least their messages to me will then be encrypted, and every additional person I get to install Signal will have an additional contact being able to use it/join a group.

This works until they either switch back to their original app or get a new phone and don't install signal on it. If you stop using Signal you need to deregister your phone number from the service, but many people don't know or care to do that.

If they don't, anyone who has ever used Signal to communicate with them will be silently unable to contact them anymore.

True. The former hasn't been a problem for me, the latter's becoming increasingly less so now that more and more people have switched, and people are starting to more consciously use it every now and then for one of the few Signal groups we have - but it certainly was a problem at first.

I did that, but it was disappointing losing my message history for SMS messages when my phone boot-looped and I had to re-flash.

It would have been the same outcome regardless of what SMS app you were using.