Information about downgrade baseband Mi Pad 4 Plus

[Davide1994]

Hi guys,
I have recently bought a Mi Pad 4 Plus LTE and I changed the original chinese ROM with the latest Xiaomi.eu (MIUI 10.3.2 ODJCNXM) with Android 8.1.0.

I have two questions:
1 - If I install a custom ROM with Android 9 or 10, can I later come back and install MIUI 10 with Android 8? Can the baseband be downgraded by flashing an "old" ROM?
2 - Do you know a good rom with good battery life and without bugs?

Thanks in advance,
sorry for my english,
Davide

EDIT: device came with unlocked bootloader and I have installed TWRP to flash the actual ROM