Solutions
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gilarelli's post in Best/easiest way to downgrade kernel (if possible without reinstalling) was marked as the answerIt's solved !
So: after modifying the /boot/armbianEnv.txt file and booting in maintenance mode, I installed the same packages as the ones on the SDCard (*rk3399 instead of *rockchip64) and now it's working fine.
I'll just check if everything is really fine, but this topic can be closed.
Thanks for your help !