
FWIW when my Whippet was around 7 months, I wanted to book her in to be spayed as I had an elderly entire, and not well, Basset at home who didn't need to be around (as in under the same roof) as an in season bitch. My vet persuaded me to wait until after she'd had her first season, citing bone development problems (growth plate closure) and the possible risk of spay incontinance if she was spayed early. I did, but by 11 months there was absolutely no sign of a season starting, so I asked him to go ahead. Her mother and half sister didn't have their first seasons before around 18 months.
She's fine.
In your case, with a bigger bitch, I'd let her have one season, personally and then book a spay for around 3/4 months later. Usually spaying is ok to be done mid-way between seasons when the repro. organs have gone quiet. Of course, if this will be her first season, you won't know how she's going to be cycling (ie mid way between seasons).