an Android or LineageOS device

You need to decide whether you're okay with Android, or want to be able to also run LineageOS. The latter will reduce your options to devices that can be rooted and have their bootloader unlocked - so Xiaomi, Motorola, Nothing, Google and Fairphone - and that too only some models (the ones for which a LineageOS ROM has been developed).

at least 6GB of RAM

Any midrange and even many budget phones have 6GB RAM nowadays.

capable of running without Google Apps

Do you just want to not use Google Apps, or do you want to completely remove them from your phone? Again, the latter will need rooting and bootloader unlocking.

compact enough for portability open to a tablet

Tablets are not very portable.

performance is key

Probably avoid Google Pixels then. They use Tensor chips, which have poor performance.

USB-C DisplayPort Alt Mode

Which devices have this, and which don't, is very arbitrary. You'll have to check for each model.