I largely limit myself to ebooks or audiobooks now, which generally means I’ve no need to purchase a book before the very second I intend to read it, unless I expect to be somewhere without internet for a while and need to predownload stuff.
If there’s a book in interested in but won’t be reading right away, then I put it on a TBR list. Previously on Goodreads, ATM I’m using Storygraph. Then when I’m looking to read something new I skim through the TBR list and pick something off of it to borrow or buy. In that sense it’s less a backlog and more a menu lol.