I'm conducting a test. Just repackaged miui 10.3.1.0 EEA by removing the system, boot, dtbo and vbmeta images. Slimmed the vendor overlay by removing everything related to china telecom and such. Flashed via TWRP over Floko and Okita kernel and the battery drain during calls seems a bit better now. I'll let you know if it's safe and working but I plan to push the package in cloud and share a link soon. If this works I'll make sure to publish updated flashable vendor/firmware packages to use with GSI roms. I'll keep you updated with the results....