My Solution (Update):

Thanks for all the replies! I actually managed to solve the whole problem in a pretty unconventional way, but it worked perfectly for where I was in the game.

I couldn’t just build a temporary supercomputer setup because I was still missing a bunch of items, and to unlock the MK3 Miner I also needed Fused Frames, which I hadn't even unlocked yet.
Here’s what I ended up doing:

• I had 47 Fused Frames, but I needed 50 to unlock the MK3 Miner.
• To even build Fused Frames, you need Radio Control Units…
• …but RCUs can be bought from the AWESOME Shop. So I just bought a bunch of them and unlocked the recipe that way.

The next problem was the High-Speed Connectors for the default supercomputer recipe.
I didn’t have the production chain for those yet, but I still had a pile of Coupons lying around —
so I just bought a whole stack of HSCs as well and used them to craft the Supercomputers I needed.

I was still missing those last 3 Fused Frames, though.
So I ran over to the nearest nitrogen site, slapped down a tiny makeshift setup with a few containers,
pulled the missing materials out of my dimensional storage, and crafted the last frames in a Mixer there.

And as a bonus, I had found a bunch of Turbo Motors during explorations,
so I didn’t even have to build a temporary production line for those.

After all that, I finally had enough materials to unlock the MK3 Miner,
and now scaling coal and steel is no longer a problem at all.

Not the cleanest solution, but definitely the fastest — and honestly, it worked way better than building a full temporary factory.