I am just going to respond to what you have asked about playing with other children first and then post separately about the sources of help which you have mentioned.
I think that with the other children you are doing exactly right if you swoop in as soon as he gets physical with them, and whether you can intervene but allow them to carry on playing or take your dc away will depend on the circumstances and also how your dc is reacting to your intervention. If you are able to be near enough to watch interactions all the time, you will notice whether or not the other child has snatched a toy or done something to enrage your dc or whether your child has seemingly attacked the other child without reason and obviously how you deal with each scenario will depend on this sort of thing. You might sometimes be able to intervene before things get out of hand, or intervene with ideas to prevent things getting out of idea, eg saying "no one is playing with that - why don't you play with that" - it depends on how well he is accepting guidance from you.
Basically the same theories apply (imo) as those I gave above - you remove the hands and explain hands are not for hitting, you stay connected and talk about the social situation with him, and you also think about the underlying feelings he has, anger, and how you can help him with that.
Ahaparenting.com which I linked upthread and mention above will have a lot of guidance about this for your dc's age group as this is the age when children are starting to learn to interact socially with each other more and to have some control over their emotions. This will be useful for you here with ideas and guidance and day to day things, how to handle it when they don't want to get dressed for example, and it will also help you build up some knowledge about the wide range of "normal" vs "not normal" so that you are clear about when to seek professional help over the years. It will also help you to not do things which may in fact make things worse - and I personally would put the "consequence" you referred to in that category, I agree with pp that he is too young to understand, and I think it might make him angry and there are better ways to handle the situation. A good way to find something on aha quickly is to google “[your question] ahaparenting” and that usually takes you straight to what is there
There are a lot of other ways you can get social interaction for your dc, though limited by covid. I think looking for a new park is a good idea. If your dc has bonded at all with any kids it is worth talking to their parents to see if they want to meet up in a less crowded part of the park for a runaround, games with balls. Subject to covid there are a lot of parent child activities aimed at his age which may work better for him, such as circuit gym, group swimming with mothers, dance - the fact that these are more structured might help. Again it depends on how he is in these situations at the moment. There are some really good play groups who advertise - again I am not sure what the covid rules are as my dc are now older. I think having a cram packed day with lots of different activities works best for this age - like searching for dinosaurs in a forest followed by play dough play at home etc.
It just depends on how or well he copes with the various different things but NB how he copes first time will be different from how he copes subsequent times so don't necessarily give up on things just because the first time is so traumatic for you that you want to run and hide!
Penelope Leach who is a psychologist wrote "Your child 1- 5" (not exact title, something like that) many years ago and has updated it from time to time, and it has a lot of really good ideas about social interaction and ideas for activities and care needs for this age group and also explains the reasoning by reference to the child's stage of development.
It sounds like you are doing brilliantly and I am very sorry about length of this.