Welcome @Bonbonkocher, have you checked devices’ XDA community ?
https://forum.xda-developers.com/f/samsung-galaxy-tab-a8-2021.12485/
As you can see, seems that - actually - GSI is the way to go:
BTW, if you wish for a “native” build, you need to seek comprehensive (re)sources for these devices:
https://github.com/search?q=Samsung+Galaxy+Tab+A8&type=repositories&s=updated
https://github.com/search?q=SM-X200&type=repositories&s=updated
https://github.com/search?q=gta8wifi&type=repositories&s=updated
https://github.com/search?q=SM-X205&type=repositories&s=updated
https://github.com/search?q=gta8lte&type=repositories&s=updated
Last but not least, reformatting the request like this one could better help builders.
Hopt that helps !