Debating whether I need to buy a second seat for a flight

I'm going to be flying with my service dog in a few months, and I'm trying to decide if I should book an extra seat for him to have more space. My SD is about 90 pounds and while he has flown before, it was a short flight (about 2 hours). During that flight, he shared the footspace of both my partner and me, and we were in bulkhead seating. For this trip, we would also purchase bulkhead seating and he would also be able to use the footspace of the person next to me along with my own, but the flights will be longer--a 4.5-hour one and a 4-hour one with a layover in between.

I'm worried that he will be too uncomfortable in such a small space for that long, so I'm considering buying a second seat next to mine for him to have the footspace of. It's expensive, though, so I wanted to see if anyone has experience traveling with a dog around mine's size for longer flights. If you have, did you get an extra seat? If you didn't get an extra seat, do you wish you had, or was your dog comfortable enough in the space you had for that amount of time?