You just need to flash a Treble enabled ROM, like LineageOS, to get Treble support on your phone.
Yes, it's possible to flash the official GSI, but probably you'll encounter a lot of bugs.
You can't recreate something that never existed.
The /cache partition is not present on AB devices.
Source: https://www.xda-developers.com/how-a-b-partitions-and-seamless-updates-affect-custom-development-on-xda/
I replied your PM, but could you please tell me which release date you were using before updating?
That problem already happenned with other people, you can updated the ROM flashing with TWRP without a problem.
You came from which version?
You have to use blankflash to enter bootloader and flash gpt.bin from the stock rom, after that you'll be able to boot normally.