[Xiaomi Mi Pad 4 Questions & Answers] NexusRunner:
Thanks everyone! Added 2 lines to /system/build.prop and UI is working like a charm:

ro.sf.lcd_density=280
ro.build.characteristics=tablet

I can also confirm that directly flash GSI system.img on stock ROM won't boot, it throws me back to bootloader. Flash Universal DM-Verity and ForceEncrypt Disable on stock kernel solves the issue. Flash it just once, we can switch between GSI ROMs without boot problem.

You don't have to get your device unlocked to get a system file, it would be easier to extract it from downloaded factory image. That's what I did because my system partition is flashed to GSI rom :D

I did some tests, flashed RR AOSP Lineage on both MIUI CN 9.6.7-Stable and 8.7.19-Beta, they all have the same flicker/brightness issue. logcat got a lot of these:

07-03 06:43:23.481 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:24.176 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:24.724 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:25.177 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:25.914 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:26.178 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:27.179 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:27.325 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:28.180 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:28.565 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:29.182 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:29.434 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:30.183 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:30.236 1281 1339 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.048 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.075 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.086 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.112 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.169 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.183 2142 2142 W /system/bin/bootctl: Waited one second for android.hardware.boot@1.0::IBootControl/default. Waiting another...
07-03 06:43:31.188 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.240 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.258 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.291 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.361 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.394 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.428 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.496 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.569 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.599 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.700 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.753 1281 1300 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.803 1281 1398 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.869 1281 1748 E LightsService: Light requested not available on this device. 2
07-03 06:43:31.905 1281 1398 E LightsService: Light requested not available on this device. 2

Besides the brightness issue, the system time always revert back to system image build time when it reboots, while TWRP seems to handle system clock and brightness correctly.

Apart from that, Camera is working, Bluetooth is working, WiFi is working, Audio is working, LTE (for my model) and fingerprint are N/A. Everything else is pretty much usable.

I'm still trying to understand Project Treble but is this a problem of /vendor or /system? I don't know if it's related or not, I heard something of "overlay" and saw some device specific apks on GSI /system/overlay/treble-overlay-[DEVICE].apk but there's no apk for our device yet. Is this something we can improve? But why does GSI need device specific config?

I'm kind of stuck here. What to do next?