Most are.20a. You can split to 2 if your loads are fairly balanced to each side of your circuits
5 kW is 2.5 kW / pole, which is ~20A at 120V. Circuits are only to be loaded to 80%, so 80% of 30A gives you 24A of actual circuit ampacity, from a continuous current carrying standpoint.
Yeah, 20A will probably work, but it isn’t what you want to do if you’re setting it up right.