Hi @eledor,
TLDR;
I only just noticed you are aware of the community ^^
I would directly install on Rocky 9, just because you won’t have to go through the leapp upgrade then, from my test environments it seems stable enough, and if there are still any very minor issues, they should be gone in 3.12/4.14.
The long version:
EL 8 is currently making it’s end of the lifecycle to run Foreman/Katello on it, there is already a thread about dropping the support in one of the next versions (Drop support for running on EL8 with Foreman 3.13).
It’s a pretty fast switchover from EL 8 to EL 9, so there might still minor bugs in the EL 9 version due to the newer OS base (i.e. Product discovery yum repo), that are not apparent in the EL 8 version, but either they already got patched with the patch releases for 3.11/4.13 or will be fixed in 3.12/4.14.
You still can install it on Rocky Linux 8 now and then do a leapp upgrade to 9, soon when 8 is not supported anymore.
But it might be better to directly choose 9. At least that’s what I would do, because it seems to be more trouble to upgrade via leapp compared to having a few very minor bugs until the next minor release is there (which is rather soon).