@Joluitsme 

What you plan should work in theory, but Ethernet over powerline is very dependent on type and specifics of wiring. If that won't work for you, you might get stuck with P9 you don't have much use. Also, when possible, it makes sense to have same type of mesh equipment. TP-Link says you can mix, and they are right, but it is always "up to the point."

In your case, your objective is to connect garage with the house using Ethernet, and attach Deco node to the Ethernet in the garage.

For that, I would suggest you buy M5 tri-pack and a pair of powerline adapters, either from TP-Link or whatever ones are with good specs and on sale. Set powerline adapters in house and garage, "borrow" one of M5 to place in garage and connect to adapter. M5 will see it as an Ethernet and use it as a backhaul. Objective achieved.

If you find that setup works and you are getting sufficient WiFi speed in garage, just buy additional single unit M5. Out of all Deco models, TP-Link sells M5 as single units. 

Perhaps, you'll find that two M5 were just enough for your house, then you won't have to spend extra on Deco node at all and your only expence will be pair of powerline adapters.

Benefits:

1. Consistent Deco mesh hardware;

2. Flexibility: if Ethernet over powerline does not work, you may still have other options to try, without constrains of powerline capable P9 unit;

3. Cost: a pair of powerline adapters and (possibly) one more M5 would probably cost less than a pair of P9s.