It's just so hard isn't it!

I have a friend (our boys in prep together) who's boy only turned 5 in May, started school February. She had him assessed, saw school psychologists etc and they basically said to send him. He is very confident socially, and is OK academically but will pass fine. My DS an October baby is 7 months older and isn't as good socially but better academically. Their
Personalities have more to do with how they're getting by in school.

Another friend with a boy in the same class is actually supposed to be in year 1 but it was recommended he repeat kindy. He had a hearing problem which wasn't picked up so didn't do well during kindy. The 3 of them are friends and are all at different stages.... None of them struggle though.

I do think that their personalities have a lot to do with it. My DS is the most academic of the 3, yet not the eldest, but isn't confident in ways they are. Each if them are so different.

Go with your instinct as you know her best. I think as long as you're doing what you think is best for her, then starting earlier or later won't make that much difference iykwim.