...this game I'm doing now is a Conventional Start with no Starport and no Shipyards. When I built my shipyards the first one was a Commercial Yard which I got to 30,000 Tons to build my Pre-Cryo Colony Ship, so that refit wouldn't fit. My second yard was naval, but only 3,000 and it was for my Geo/Grav Survey Ship. I've just expanded the slipways on my existing yards and built up another two, one Naval and one Commercial, and I am in the process of expanding these new yards to 9,000 Tons for the Naval and 90,000 Tons for the Commercial. Sometime after I had finished converting all 1,500 of my Conventional Industry, and right about towards the end of expanding my TN Industry I began to have CMCs popping up like nuts. My Mass Driver was still queued up and I had to wait for my Starport first for Role-Play reasons, so I built some Haulers for the interim. They got about 10 years of use, so not too bad, and I'm in the process of replacing them right now.
Ah, yes, the small version makes sense as an early mineral mover for a conventional start.
I only ever build dedicated mineral haulers to alleviate the need to use my big freighters. Even then I hardly build much or any of either as I typically rely on Civilian Shipping for 90% of my installation moving needs and Mass Drivers for my CMCs, so typically a 5-10,000 Ton Freighter or two will do. At that point I just fold the freighter into an Orbital Miner and be done with it, since with a little extra work I can use the same design as a Salvager by just swapping modules.
I only build big freighters, because I find that Civvies are too slow to respond to contracts to move my installations around (and occasionally mess things up, if I create several demand contracts at once).
This means that I spend a couple decades in the early game fully utilizing a freighter yard, while adding more slipways the entire time.
I end up with hundreds of Haulers.
And while 25kt of cargo space is overkill for small mineral runs, I'm happy to live with the occasional overkill rather than spin up a separate yard for a freighter optimized for mineral hauling.
My mineral mule routes generally cover multiple systems, going empty outwards from Home and then scooping at a single collection point in each system on the return leg. A single Hauler can handle each route for a while until total mineral extraction along the route exceeds Hauler throughput. It is convenient to not have to worry about smaller throughput increments. I just check stockpile sizes as part of my annual New Year checklist. If minerals are backing up, I add a Hauler to the route.
This will be my primary intrasystem mule. It can move six units of Infrastructure, or one 25,000 Ton Installation with 5,000 Tons to spare. A good ship for the kinds of odd jobs I intend to have it do.
Good news: infrastructure is half the size you think it is. 2,500t. One standard hold can fit 10 units of infrastructure (normal or low-grav). Your design can fit 12.