Seedlings should be transplanted when they have their second set of real
leaves.

How will I know when they need to be transplanted a second time. I
think this may be the case for tomatoes and some of the other larger
plants that I start. I suppose if they get root-bound, it's time to
repot...? On the other hand, I've bought healthy plants from
nurseries that are basically a small pot of solid roots.

It's not clear when you started them but these seeds are slow to germinate
so you should be ok as long as you give them lots of light (so that they
don't get spindly) and harden them off properly in a cold frame of some
sort.

I started them about 12 weeks before last frost. I agree that they've
been slow to germinate, my Blue Spruce seeds are sprouting faster!

Great to see a message from another gardener in the great north, eh.

Thanks,

- Shiltsy