How to root iodé os 3.5 on a Samsung S9+ G965?


I need root to downclock the speed of processors with governors with Kernel Adiutor ( it reduce the consumption by 2 ). I’ve done it on my Sony XA2 pioneer with iodé os 3.4, 734Mhz instead of 1200Mhz in low power consumption ( it’s stable, and okay for web surfing, no bugs this year ! ). I think it will also increase the life of motherbooard and it"s battery.
When I am fliming HD, taking picture full resolution, composing music or editing video I switch the best frequencies clock governors.

I’ve patched the boot.img with Magisk, but I don’t know how to load it in the phone.
Does I need to copy the rooted boot.img in the OTA then zip it again and do again the installation process ( by reflash stock firmware with ODIN ) ?

I very not sure that it will work, because the patched boot is different ( also in size).

Does someone get this phone rooted with iodé os 3.5 like this ?
( I dislike android 13, and S9+ is faster on 3.5 )

Check out the official guide for Magisk, and Samsung devices in particular (they look more complicated than other devices, never had one though):

Boot.img extracted from the original rom of iodé 3.5, then patched with Magisk but I don’t know how to install it ( my bootloader look fully unlocked in the Download mode ).
My problem is that I have no access for minimal adb fastboot in the recovery of iodé os, only sideload is working.
So I wonder if I can put the boot.img patched in the unziped iodé os installation rom, then re-zip it and flash it with adb sideload. i don’t want to brick the phone with a detection of a modified boot.img, that’s why I am asking…