My knowledge is really out of date now, but I lived in one of the small villages outside Bethesda and did my secondary schooling in Bangor through the 90’s. I really wouldn’t worry about the location ruining your kids future. The bright kids with motivated parents always do well, my school gang are now vets, doctors, barristers, professor at Oxford Uni etc.
It was a great place to do my teenage years. Yes, I had to jump on my bike to go and see my nearest mate 2 miles away, and the winters were sometimes long, but the memories that hang around are all on bikes, or climbing days, or mucking around at the lakes. On balance all positive.
I didn’t really get far with Welsh, but if you get stuck into the PTA/clubs/community things being English is quickly forgiven.
If our careers allowed we’d be back there like a shot.