besides which copper price is down, run a new heavy whatever cable
like a 6 gauge to a sub panel and save yourself a lot of hassle

That is what I have previously done, 6-gauge to a subpanel.
Initially, I only brought out two 120V circuits for outlets, and a
220V circuit for lighting. But, it's getting *really* cold, and a
friend will be working in there with me. With multiple lathes,
halogen work lights, and space heaters working at the same time, it
was time for more outlets.

Now, with 6 20-amp circuits for 120V outlets, I'm more than set. If
I needed more than that, I'd need a bigger run to the subpanel - and
since I only have a 100A service on the house, pulling more than 50
amps just to the garage wouldn't be that good of an idea.

Doing one splice in each box with the 10-gauge wasn't bad at all.
Everything went quite well, with no surprises.