" I’m worried about other parts failing due to the increased pressure of a new seal - is there precedent for this after a head gasket repair?"
No, you really don’t have to worry about that unless you only have one head done. If only one side is leaking and only it is repaired it is possible the other side may have a problem later on but then it could go on without any trouble. You just don’t know for sure unless it is redone also at the same time.
Since you really don’t want to have new car payments but want good reliability I suggest you look into replacing the engine with a totally rebuilt unit as I suggested in my first post. It will cost more than just having a headgasket replaced but you will have a 3 year warranty, on the engine at least, just like a new car would. You get the benefits of having a new engine at a fraction of the cost of a new car.
It is possible other things could be a problem later on but going on Subaru’s past history for these models you are facing the most common complaint with them, the headgaskets. Revisions in the gasket design have solved the problem now and things are better. The rest of the vehicle shouldn’t be a problem. The CV joints may need to be replaced later on but the replacement cost isn’t too bad to have that done. Even though the mileage is getting up there on the car I think you will be good to go with a rebuilt engine installed. You will also get a little better price for the car when it comes time to sell it.