Well, at least you're sorry, so now that it's here, here's my very personal experience from A11 to A12...
[ This is going to be an aparté and I'd request you not to reply back with more questions or request for more details, we'll keep it to this, thanks]
Tbh, the process itself of going from OOS A11 to OOS A12 was not as straightforward as most people claim/experienced. First, the process was stuck at 99% forever (and I mean forever, not like just 1 or 2 hours, I kept waiting for half the day for it to complete, to no avail). I was then still on A11, with probably a half-broken updated system that would get stuck on the next boot, so I didn't try to install Magisk on partition B or anything such, I just disabled all Magisk modules and attempted to restart my phone. Surprisingly, it seemed to reboot correctly and finalized the installation. However, of course, my system was now unrooted, so it took me more time to manually patch my new boot.img and make sure it was properly rooted.
After all this, I could really start to use this new system and see both its cool sides and limitations (and bugs as well, as I experienced quite a few). I used to get a temperature alert from the UI quite frequently on A10 & A11 but not anymore since I updated, which is quite pleasant. The battery life, once rooted, all ads disabled and network communications & processes strictly limited, is surprisingly decent as well (I was expecting worse, from what I read), quite similar actually to my tweaked A11 OOS. I could also notice a more polished UI, smooth and quite pleasant to use, with fast animations/transitions (speed can be adjusted in the settings anyway) and lots of freedom and control given to the user over the system, security & apps. Oneplus new Launcher, however, is a pain and really needs to be improved and fixed (impossible to remove the left page of the home launcher, for example, which I later found a "fix", using some Oppo apk), folders and icons cannot be renamed nor changed, there is no more way to hide some apps by 'stretching' the screen, and I am experiencing some serious issue with the light sensor under low light (when in auto-brightness), as it set my screen to the lowest possible brightness setting, which makes it barely visible, unpleasant to read and transforms the colors as well. There'll probably be an upcoming fix for this bug, but for now, it really s*cks.
Overall, and after some tweaking, it's still worth it, not just for the more recent security updates. I miss the overall 'stability' of OOS A11 (after all the updates they've made to it for the past 2 years, have to admit) but knowing that most apps are now being developed with A12/A13 in mind, it is probably a safer choice to stick to A12+.
Cheers!