The first bitch I had speyed, was about 6 owing to a mammary lump. We waited as long as we could but as it became apparent that it was getting bigger with her season approaching, I caved in and got her speyed.
She was always an ambitious bitch, and we'd had to be careful not to let her boss us around. Couldn't even allow her on the sofa or she'd get 'uppity'

Afterwards, it became clear that she just didn't want to take over anymore and we could enjoy cuddles with her on the sofa. She also became much more likely to listen to me when out on a walk, which was lovely :-)
It didn't appear to affect how my three bitches interacted. They'd never been moody, not even with all 3 being in season more or less together/one after the other. Nor have they ever been moody with a phantom, though they all had them at one time or another.
Interestingly, when we lost her last year, it became apparent that she really was the matriarch, top bitch, whatever you want to call it. Though she never so much as curled her lip or even grumbled at her pups or her grand pups. I always felt she was far too permissive with the youngsters, but clearly she was in charge. The remaining two were a little lost at first, and it's taken a year for them to adjust. But they are fine together as they always have been.
My youngster does miss her gran though, they were the chief cause of frolicks and meyhem, her mum has never been one to play with other dogs, she prefers to play with and cuddle her humans.