Everything worked like a charm.
Coming from a stock and updated EU 188.8.131.52.LE15BA I first unlock bootloader really easily. Coming from an old HTC U11, the O+ unlocking method is just sooooooo easy and shorter. You may pay attention to the fact that, once bootloader unlocked, adb reboot bootloader command isn't working and just make OOs reboot to itself. At least on my device, I have to manualy enter fastboot using vol- and powerup at boot combo
I then did boot from a Magisked (BA / DA) IMG but once landed on OOS, magisk manager asked me that it needs a specific step for my device and then reboot. Then I update magisk manager to latest available stable apk, but from there, MAGISK won't let me flash to the available slot. So specifically here, I needed to re do the fastboot boot boot.img command : the update + reboot MAGISK needs get me out of the temporary BOOT.IMG I was on to a true boot.img.
Once rebooted, magisk manager now allowed me to flash to the available slot and everything now works flawlessly and safetynet is 100% ok once Magisk hide is triggered.
That's why I choose O+ to replace my old HTC U11. This latest had a good community to magiskroot it and I knew many now chose O+ because of the community developments on their device.