I'm wondering about putting a subpanel in my garage.

Right now there's only 1/2" EMT under about a 10 ft run of concrete and
one 15 amp circuit.

Question 1. Would it be legal to run three #10 wires for a subpanel fed
from a 30 amp breaker and leave the #14 for the 15 amp circuit? If that
conduit is continuous will that be fairly easy to pull through? Can I
ground through the EMT? I've heard this might lower the allowable
number of wires in the conduit.

Question 2. Once I have that subpanel, is there any limit to the number
of circuits that can be split from it if none of them are over 30 amps?
eg. could I have a 30 amp 220V and a 20 amp 110V?

Thanks for any help you can provide.