Keep root between firmware versions ?

I have a Meizu M3 phone, which can be rooted only in older firmware versions using the "normal" meizu procedures.
As it is very easy to downgrade versions (back to an older "rooteable" one - 5.1.4.0), I've made my mind in doing some tests.

I've decide to give a try to flashfire to help me go from a rooted (but older) firmware version into a new version without loosing root:
I've choose the update.zip file of a newer version (5.6.8.30), configured everRoot to inject SuperSU into system and tried 2 approaches:
with and without mounting system R/W.

In both approaches the system is upgraded.
su is available in shell (# sign, after calling it).

But the system is NOT really rooted :( - tools complaining about no root, ES Explorer can't recognize the root, flashfire stays forever waiting for root access, and so on ....
Also, when trying to mount system as RW fails:

su
mount -o rw,remount /system
"Operation not permitted"

I ask the forum for suggestions:
Is there any suggestion with flashfire to help-me update versions without loosing root ?
Alternatively, should I repack update.zip removing both boot.img and recovery.img (editing also the update script) ?

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