Porch remodel with improved electrical service
Nope.  You need a 20A breaker at the main. 

When you upsize wire for long distance, you do not also upsize the breaker.  There's no reason to; the long distance doesn't consume any additional amps, it consumes volts. Amps remain the same. 

A 30A breaker is not legal on a circuit supplying 15A or 20A breakers, you can only have 30A receptacles on that.

If you really want to get 30A worth of use out of the #10 cable, then add a subpanel at the porch, and you can have 2 or more 15/20A circuits coming out of that sub.  You are allowed to install a 120V subpanel, and that requires a 3-wire connection.