For anyone coming across this issue in the future, I finally managed to find a solution for the hotbar problem after contacting Larian support with a bug report, they were a great help! What I had to do was first disable steam cloud saves for BG3. Then go to the BG3 subfolder, the one that contains data like your save files, and rename the folder. Then you boot up the game and create a new playthrough. Check to make sure you can create saves with no issues. Then you transfer over all of your saves from the old, renamed BG3 subfolder, into the newly created subfolder. And whalah, hotbar working again! Bear in mind this is a gross summary of what made my game work correctly, there are many more steps & things to watch out for if someone else is having the same issue, as they might have a different PC situation! Big thanks to the Larian support team!
