Everyone who gives you advice is seeing the situation through the lens of their experience and expectation. It may be that the preschool teachers are simply looking at a lovely little boy who enjoys his play...so they want him to play. You are seeing a beautiful but troubled son who has needs you know will not be fulfilled in preschool, or kindergarten by the sounds of things.
I wasn't quite clear if you had fully pinned down the source of his anxiety, depression and frustration but no doubt an unfed craving for knowledge will contribute to that.
You have good data from his independent assessments and your knowledge of his capabilities. You have had good recommendations from a psychiatrist and a positive conversation with a school principal. These are all massively important and a place many would like to be able to reach.
If you are comfortable with the school where you have had discussions then continue them and look to whether kindergarten or first grade is the better opening spot. As you have said, he has 100% mastery of kindergarten so it won't do on its own for academics. It may be a softer start to school, in that he will be learning about the school environment, rules, where the bathrooms are etc, so look at those pros and cons. It may be possible to spend part of the day in kinder and part in first grade while you work out where he fits better.
Finding likeminded peers in kindergarten may still not happen. Certainly being with older kids who can hold their own with your son in play will help with his tendency to dominate. That is not uncommon in gifted kids who want more complex play, complicated rules or imaginative elements NT kids aren't necessarily ready for.
There are checklists that are used to assess children for acceleration. The Iowa Acceleration Scale is one. I have seen others for early kindergarten entry. But, these children are very individual, dare I say, and a template or process doesn't necessarily work.
It is hard and it is scary to be in control of the advocacy for your child but this is only the beginning. There is no better advocate, no-one who knows your child better, than you. Seek advice, do lots of research and reading and ask lots of questions but the path you choose will ultimately be yours.
I have often wished there was a path, a process, a known well lit way forward but it never appeared. I have spent a lot of time beating a path with a machete guided by the light of lots of internet research and reading and thinking and the knowledge of others who have forged similar tracks in the jungle such as those on this board.
Best wishes and know you are not alone in the world but you are probably alone where you are for the moment. Don't be scared to trust yourself as you will become the expert your son needs.
