"Has anyone else been in this position and got any advice?"
I think my situation is the same as yours but I'll tell you how it went for me.
Ds came to us at 3 nearly 4 in late spring almost 7 years ago. We are in England.
Da was due to turn 4 in the summer and a few days later school expected him to start in Fundation.
I was not happy about this and wanted him to delay by a year and start Foundation at age 5. Kiss do not need to be in school until the term after they turn 5 (unless home schooled of course).
Initially, school seemed open but in the end all they would offer was if he started at 5 he would go straight into Year 1.
We arranged a meeting with adoption Ed psych. She suggested he start part-time later and then go full time before end of Foundation.
Thus is. What we ended up doing. He joined part time in March and after the half term in May he went full time. It meant he had nearly a year at home before starting.
He is very bright and it all worked well for him. I did not want him to go straight into Year 1.
School implied (possibly wrongly) if he defered a year that when he went up and to secondary school they may not respect the cohort and may jump him up a year.
It worked ok for him. I know two friends with kids in his current year and he has managed well. At the time it was hugely stressful but in the end we took Ed psych's advice. And all has been fine.
This bass on my son and may not be good advice for you.