The real answer to your question is: Go buy a new phone... Let me somewhat explain.
In the ROMs I found section of your question you seem to be overly optimistic about all the ROMs available, but you are simply mistaking. Let me go through the list one by one.
Replicant: Available for the S2. But the latest update is from September 2015, build replicant-4.2-i9100.zip which is built on the ancient Android 4.2 Jelly Bean. And even that old build is far from completely functional as it is missing (see Replicant status page) 3D graphics, Bluetooth and USB audio, WiFi and Bluetooth (without non-free firmwares), NFC and GPS.
LineageOS: Is not available for the S2. It has an automatically generated landing page for the phone with some info on it, but here is the builds page https://download.lineageos.org/i9100, and it is empty. (oddly enough I see on Web Archives that there was a recent nightly build available but it is now removed. Might be it was too rough.)
OmniROM does have a build for the S2. but it is from May 2015 and -As Izzy pointed out in the comments- based on Android 4.4 KitKat.
AOKP though available for the S2, it is only available as a nightly build (which is officially unfinished and has bugs) and while the latest build is aokp_i9100_mm_nightly_2017-01-20.zip which seems to be quite recent, it is an automatically generated build every few days and mustn't be it has changed anything since years ago.
If you do want to use this device and looking for the best available, my recommendation would go to the CyanogenMod! While they are officially dead their work may still be found if you really want it.
They have a 12.1 Stable Release (download it here from Web Archive) from late 2015, but while not that much newer from the two available above it is built on Android 5.0 Lolipop, so at least the Android version isn't that ancient... Even better would probably be to use their latest nightly build, from December 2016 (on Web Archive here) which is version 13 and is built on Android 6.0 already. (more info about that nightly build you can find in its page here.)
There are some other, lesser known, options. One of then is Resurrection Remix ROM which is based on CM but is very much fully up to date based on Android 7.1! More options may be found by looking around in the aforementioned Xda-developers forums section for this phone https://forum.xda-developers.com/galaxy-s2