Hi tutop and welcome to the community and the forum.
I’d say the most important security aspect is the frequency of Android security updates. Stock FPOS (for the FP4) currently has 5 August 2022.
However, if you take reasonable precautions you might find it’s more important to keep Big G (and doubtless partners) out of your system. Moving to stock FPOS you’ll be accepting what some people would see as major privacy issues, even if your only (visible) contact is through the Play Store – and that’s unlikely to be the case. After all, that’s why alternative distributions exist.
I have no experience of Murena on the FP4 but I run it on a FP2 with much satisfaction.
P.S. - If you decide to switch, make sure you know what you’re doing, read up on this forum and plan all the steps beforehand.
See here for example:
EDIT - One last thing: the privacy issues that come with stock FPOS can be seen, in a way, as permanent security issues.