If you’ve done the MR in a day, you should be strong enough to do Shasta in a day. If you’ve done Raineer, you should be technically ready for Shasta. I recommend climbing up Casaval Ridge. The normal route goes left of the crest of the ridge, avoiding lots of crappy rock. You should have plenty of tracks to follow. You can descend either Casaval or Avalanche Gulch or even Sargents/Greene Butte. Be aware that route finding can be tricky on the ridges, and weather can make navigation difficult on Shasta.
