I actually picked the OP 9 because it was supported by the next-best secure version of Android, DivestOS, which was still being supported by its developer at the time.
Unfortunately Tavi abandoned the project last December, and since he was running it alone it’s basically dead now and will no longer get updates, so its security will gradually get more and more outdated and eventually become useless. What also doesn’t help is that he didn’t put as much work into making apps that are dependent on Google libraries work pretty much like on a stock Android without compromising the overall security (Graphene has its Sandboxed Google Play thing), and since that means I have to use weird alternative apps even for the most basic stuff, I keep running into annoying limitations and dysfunctionalities that I would never be able to accept in an EDC phone.
Very probably I’m going to look into installing something relatively secure/private but “more normal” like /e/ OS, or if it’s still too weird I’ll just get back to good old LineageOS and try to harden it myself as much as possible vs. how it comes default.