At the beginning when I first started I was dead set on having like 5 specific villagers on my island and swore I'd never replace them or be happy with it until I got them. I found it made searches on mystery islands kind of difficult and disappointing and after I collected some of them I kind of stopped that approach and started taking villagers that felt right at the time.
Now I like to keep 1-3 spots for these kind of spur of the moment change decisions on each of my island. In other words, I have 7 or 8 set villagers with two spots I'm good with letting some come in and out. I think I tend to hang onto the rarer villagers or villagers that don't currently have amiibos but in general I'm okay with letting some of the more common ones go if I have their amiibo. To make the choice I usually look at how much I enjoy interacting with them, what their house looks like (i.e. does their house match my island or does it stick out, etc.), what they look like, can I get them back in the future, etc. when deciding my permanent lineup.