I won't be going for custom roms atleast till the device is supported.
@Adithya yup agree, setting up roms have become quite difficult even since I would flash since Kitkat... man those days were like a breeze,short and simple.
I don't intend to use Magisk either, for what I believe, it's primarily used for Xposed mods or SafetyNet. Not my cup of tea.
All I am trying to do is future proofing that in the eventuality(0.0001%)of setting up custom rom without rooting, is magisk necessary to use banking app/financial app. Flashing/rooting was fun back in the days for me, not anymore. If apps won't work, no custom rom for me.

But I belive a debloated MIUI should hold good for atleast 3-4 years, right? Unless,
Xiaomi deliberately slows it down would be a different case.