Notomato sauce.

Could you mix a small bit of tomato puree with stock and maybe add some chopped aubergine and/or mushrooms?

I can't really think of anything that truly replaces tomatoes in a spag bol.

You could try a white bolognese sauce - culinaryginger.com/traditional-white-bolognese-sauce/

Can’t you get your acid reflux sorted out?

There’s a magic pill called Omazaprole, have you asked your GP?

How about using red peppers? You could roast them first with olive oil to bring out the flavour, then blitz them to make a purée.
I thinks I’d cook the mince and other ingredients first and then add the purée at the end, so not to spoil it’s flavour.

Yes, as ferry23 suggests, a white Bolognaise sauce with meat, onions, garlic, carrots, celery, white wine and a good stock would be an alternative. Known as Ragu Bianco.
You could add milk or cream according to some recipes. I wouldn't but that is a matter of preference.

I’m not sure any of these would help with acid reflux, you need to cut out more than tomatoes

A green pesto? Basil, olive oil, pine nuts and grated Parmesan with a dash of lemon.
Rocket or one of the spicy ‘Asian greens’ works too. Or spinach with a dash of chilli.