How should I upgrade or combine sub-panels so I can add another one?
Honestly, from the questions you're asking and, the work you've described, I think you should hire an electrician.
If you're running 120 from one location to another, you don't need a sub-panel. Your 220/240 is two-phase. It is split in your service panel, left side breakers are one phase 120/110, and the right is the second phase. You get 220 from a panel by using a 2-gang breaker and providing two legs of 110 on the same phase. Wire rating is as follows:

15A - 14 ga.
20A - 12 ga.
30A - 10 ga.
40A - 8 ga.
55A - 6 ga.