As of ShiftOS-G Version 6.2. you should be able to reset the Rollback protection index with fastboot oem reset-spl, as mentioned here.
After that you should be able to install Android ROMs with older patch levels than your current one(not sure if there is a limit how far back you can go, but for the current one it shouldn’t be a concern.
To check which SOS-G Version came preinstalled or is currently installed on your SHIFTphone go to Settings->System->Systemupdates(or similar).
There ir should say something on the top in big letters. …6.1-G or …6.2-G
Also according to this thread probably this week, which should also ship with a more recent SPL.
If you feel unsure about flashing a new ROM I recommend waiting a bit more and to make sure everything needed is in place. I also assume that in a few months or so the offical /e/OS installer should support the SP8, which should make the installation process a lot easier, especially since the current manual is a bit out of date.