Remove personal info B4 sale

[JBMouse]

Can I ask this question which I can't find answers on the forum:
I am selling my Q5 Pro (I still have and use a Q10 Pro 6GB, so the Q5 is surplus now), and would like to remove my personal info from it before sale, but keeping the custom firmware I have painstakingly installed!
It is running Android 7, Custom firmware HMD 2.1.4 2019-06-04.100340, I can't remember why 2.1.7 is not installed, think it wouldn't install or something.
Would a factory reset take it all back to Android 5 and basic firmware, or could I do that reset and it would remove my info?
Thanks for any help.

[futeko.com]

A factory reset would wipe data on the box but leave firmware as is.

[JBMouse]

All done, as you said, firmware still in place, thanks.