I will not vouch for this to be “no risk” because I still do not understand whether or not there is a difference between the 1st of a month and 5th of a month versions of SPL, but:
I flashed FPOS with an SPL reported as 05.05.2025 and after that /e/OS with an SPL reported as 01.05.2025 and was able to lock the bootloader.
If I remember correctly, every new version of SPL changes the rollback Index. As long as the SPL of a ROM has the same rollback index as the already installed one, it is safe to lock the bootloader.
Anti-rollback triggers (and bricks your device) when the rollback index of the new OS is lower than the already present one and a bootloader lock is attempted.
It’s outside my expertise but I think it is probable, that there is a fastboot command to read the rollback index present on the device before and after flashing a Custom ROM to check that. Maybe someone more knowledgeable can weigh in on this and inform us about this.