You can't make breakers trip preferentially...
Your stated goal is to have the breaker in the garage trip before the breaker in the house. That isn't going to work. Breakers do not work that way (unless the breaker in the house is Federal Pacific, sorry, inside joke there, those are notorious for tripping way too late, but that's not a desirable feature).
... But you have a better chance if they are much larger.
Since you have laid the PVC pipe anyway, my suggestion would be to forget the 12 AWG and run three 8 AWG individual wires and one #10 bare ground wire. Then the breaker in the house can be 50A. Obviously that will tend to trip later than your branch circuit 20A's.
However this marries you to the idea of a subpanel in the garage, including ground rods. It needs to be a main breaker panel, because it needs a master shutoff switch. I suppose you can use that hypercheap 2-space panel with a 20A 2-pole breaker forward-fed, since the handle-tied 2-pole breaker will double as both the master shutoff and the 20A branch circuit protection. I caution you, however, you will find this stifling - not least when the saw overloads and plunges the entire garage into darkness!
Since you have plans to build another garage in 5 years (we know how that goes, don't we :b) I suggest getting the panel you'd ultimately use there, now, and move the panel.
When you build the new garage and need to free up this panel, you may need power in this old garage in the interim. Simply straight-wire right through where the panel had been, leaving no breaker protection in the garage at all, then back at the house, downbreaker this old garage circuit to 20A. That is perfectly legit, you don't need a garage shutoff switch for a single multi-wire branch circuit. The only reason you want one is to have breakers trip in the garage first.