He's 7 months and the redirecting is bad. I can't cut protein as the only thing (and I mean only thing) he will eat is microwaved chicken breast

He won't take treats (I've tried everything from shop bought to liver, cheese, beef). Fuss is great, but isn't rewarding enough on it's own to work on anything but the basics.

He'll kick off at 'some' dogs and also at nothing at all taking out his frustration on my hands - and not gentle bites

I don't think covid ever really went Kazz. Shame choir was cancelled but for the best.
Yes Kazz they do grow up quickly.

You cannot not give him his chicken Chris, if you feed raw, kibble or canned there is going to be protein.
I feed raw and would you believe the protein is lower in raw meats than it is in dry foods or canned foods.
I researched thoroughly while trying to sort Brewsters growth rate issues and sticking with the raw is best as he gets a range from 15% to 18% protein where as wet foods are a lot higher even chappie which was recommended to someone with similar problems with the pasterns as they wanted them to reduce protein.

Sounds like he may be reactive, he is still young so I guess work on him as much as you can.
Have you looked at clicker training and trying to get him to focus and watch you ?
Sometimes the biting can get the better of you but I am sure he will outgrow it it isn't like he is under socialised. He needs to learn some manners same as Brewster.

Chris does he wear a harness? Being that he is a smallish pup and doesn't weigh to much, when he kicks off at other dogs lift him by the lead that is attached to the harness just for a short while and say 'No' or 'don't you dare' that works instead of 'No' which is a word that dogs hear so often that they tune out of the word, which is after all just a sound to them. Say it very sharply and then when he has stopped kicking off let him back down on the ground and praise if he has stopped. If he hasn't then re-lift again. It might work. It is just a case of him realising that if he kicks off he is removed by that way of doing it. It also saves your hands. In the old days we used to use a rattly pot thrown down near to the dog and that distracted the dog long enough to stop the behaviour so that might work. Neutering will not stop the behaviour all it does is stop the unwanted puppies. Training is the key to all thing.This is just again my thoughts on what to try.