Hello, I think if you're happy to use it after the infant seat has been outgrown and you like extended rear facing, then the best option would be to get a seat which goes up to 25kg instead of up to 18kg.
You might not have come across these before, as they tend to be sold in specialist shops but they will last you much longer as they tend to fit children until they are around 6-7 years old. Suggestions would be Britax Multi-Tech 3, Britax Max Way or Axkid Minikid (the Minikid adjusts automatically for each child in the seat, but it's a lot more expensive.)
The problem with belt fitted ones up to 18kg are as you have seen the belt path is in the way. If you're going to use the seat a lot, I think it's annoying enough to want to spend a bit more to avoid it. It's not too bad for an occasional use or travel seat for example.
The Graco Extend is also a particularly short-lived seat. It's outgrown when the child is around 100cm tall, which is the size of an average 3.5 year old or a tall 3 year old. You would be better off with Joie Stages if you wanted to go for a cheaper model, as this is a much taller seat. It does however still have the seatbelt routing problem. The other issue with the Graco Extend is that when you use it for children of different heights, you will have to completely unthread the safety harness from the back of the seat and re-route it through the correct height slots for the child, whereas the Joie Stages adjusts by moving the headrest up and down, the Britax seats adjust by moving a lever on the back of the seat (which is of course accessible as they are rear facing) and the Axkid adjusts automatically when you do the child's straps up.
25kg rear facing seats like the Britax and Axkid that I mentioned use a different seatbelt path which goes under the seat, they also attach using tethers which route around the seat in front or the seat rails of the front seat. The tethers can simply be left in place when you take the car seat in and out - they harmlessly tuck under the front seat and don't get in the way. You then just click them into the seat and tighten each time you install, just like a seatbelt. The more expensive Axkid is easier to tighten the tethers on as well, which is the other feature that makes the price higher.
Just another note about isofix seats - you can make the isofix points in your car more accessible by using plastic isofix guides - these usually come with any isofix seat, but they are interchangable between car seat brands. They don't affect the adult passenger's comfort when a car seat isn't installed. They just hook into the bars and hold the seat fabric open so it's easy to access them. I've attached a picture of what they look like.