Usually Magisk doesn't worry about vbmeta, but you might clear it and try.
(Or you can use a patched vbmeta.img)
Code:
fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img
Me? I'd try to avoid swapping slots. Just flash the boot image to the current slot.
Did you extract system_a already from super? Is it in erofs? Is it in ext4?
If you PM me a link to the stock recovery I might find time to make a custom recovery.
There are still no updates for this device.