Wi-Fi boosters are a waste of money, but you should be able to add an additional TP Link PLA, which has wireless, as they all work to the same standard.
All is explained in the link I posted. Click on 'what is powerline' on the webpage.
If there's more than one consumer unit, powerline adapters may not work. So the garage might have a lot to do with things if there's interesting electrics. But your main point is not correct as virtually everyone has a single feed to the house - it's what happens after the main CU that is important.
I have a hybrid with Eero covering the main mesh network for the house, but have a wired backhaul, with one Eero attached to a powerline adapter for one room, on the same consumer unit. Generally Eero mesh is fantastic though. But you cannot fight against physics - thick walls are thick walls.