A Quick Update: the Tablet just sat indefinitely and from time to time I tried if it "allow" me to work with it again.
And then @muhomorr just wrote to me on Matrix: "You can use a USB Mouse"
I felt like suddently my Third Eye was opened or smth. How have I not Considered it myself???
And sure enough - the Mouse worked Flawlessly even while Screen still Glitching like hell. I activated OEM Unlocking, Unlocked the Bootloader, Flashed the Last Public Stock GoogleOS, Locked it again - the GoogleOS was starting... but the Screen Glitch + No Touch Issue not only Happenned again in the Exact Same way as on GrapheneOS, but started to happen A LOT more Often.
I checked my Warranty and - thank's God - I'm still eligible for a Full Gadget Replacement! Weirdly enough it says that "Pixel Tablet can't be Repaired - only Replaced"