@pigpig: phhusson has added a hardware overlay for the FP3 on my pull request. So with the next GSI builds by phhusson or AndyCGYan installation should become easier. You may want to replace steps 3.-6. simply by fastboot --disable-verity flash vbmeta_a vbmeta.img && fastboot --disable-verity flash vbmeta_b vbmeta.img - there would be no further need to enter twrp…
BTW, on the most recent 0045 firmware LineageOS 17.1 works smoothly - at least my own build which is based on the same sources like AndyCGYan’s but includes a quick & dirty OTA option.