Aaaannnnd.... I figured out last night that I don't need to go to 9, only to 8.4. Strangely, what version I needed to upgrade to from 8.0 was not stated anywhere in the mass of emails Amazon sent me. I hadn't yet gone on RDS and noticed that 9 is not on the list of options, I was just trying to build 9 on my Mac.
8.4 won't build in homebrew under Monterey, though, so I'm stuck with 8.3 for my dev stack. I guess I can live with that. I'm dreading the next forced upgrade.
And latest 8.0 and 8.4 is supported at least a year from now.