You'll progress in the Portal activities by doing Master difficulty or above. Quickplay solo ops and Quickplay fireteam ops will get you to 400. As for a complete reset, no, if you still have the lower items or pull 200 power items from collections - you can delete the higher power ones (300) so it brings your account power back down and that'll let you progress from 200+ naturally but you're just going to end up progressing back up to 300 through a lower difficulty and then end up back in the harder difficulties again.