The lowest star rating I've seen since beating the game is 3 stars, but there are some 1-2 star stuff that I don't have yet (such as Sinistea). Is it possible to spawn those somehow?
Unfortunately, it is not possible spawn 1 or 2 star raids after you finish the story. Raid dens will only spawn 3+ star raids.
I have confirmed this by playing 100+ hours after finishing the story, and never encountered a 1 or 2 star raid (except for those appearing in the Y-Comm). A few Youtubers doing raids have confirmed this as well.