Reception is mainly learning through play - OP, most school receptions are happy to have parent volunteers, have you thought about volunteering in the class? You can see whats happening.
If your dc didn't do preschool, then yes, they will take more time to settle without getting tired, its so stimulating for them, so much to learn, and if not used to being around large groups of other similar aged dcs, that alone will be tiring.
But if you dont think the standard state school is right for you, have you explored other options? There are some forest school based private pre-prep schools near us that go until year 3. Private schools are usually smaller classes/higher staff numbers per child. It wasn't financially an option for us, but if it is for you and you don't want to home school, worth exploring.
Both of my dcs were more than ready for school, but dc1 is a winter baby and could already sound out words before starting, dc2 is a summer baby, but is so social compared to dc1 and wanted to spend all day with other kids, not me ! Both dcs had other friends who clearly weren't ready for another year. It is a shame they don't do intakes at January and April anymore for those who are younger and not ready.