What's stopping you from making a 10,000T ship with 3 reloads and 10,000 MSP? For colliers/supply ships, increasing their size doesn't truely benefit you once you get past 5,000-6,000 tons - since you mostly stop benefitting from engine size increases at that point.
Though you are probably overstating supply needs. To be honest, I don't think I've even used a supply ship for dozens of games. It's too expensive to keep ships on deployment through supply deliveries and for battle damage MSP shared from other vessels works fine. Pretty much the only thing I use supply ships for is to cache supplies on a forward survey base, and you don't need much for that.
I'd go down to 3000-6000 supplies and use the rest for magazine space. Whether a collier needs to be fleet speed or not entirely depends on how you are using it. If your carrier has 5 reloads built into it you probably don't need battle-line colliers IMO, but that's a subjective judgement.