Hi Annie! Welcome to the OB.
Your Phals sound like they are doing well, so congratulations! I've never heard that MiracleGro is a no-no, but there are orchid-specific fertilizers which are going to be better. Use them weekly and at quarter-strength and you should get good results.
Regarding the flower spikes, I'd advise not cutting them back. Let them do what they will. Some naturally start a new bloom cycle, while others don't, but let them do what is natural for them. If they stop blooming, then rest, then start a new spurt, enjoy it. If they stop blooming, then begin to yellow, cut them back completely.
I'd also urge you to turn the light off at night. Orchids really do need a rest period of darkness. If you can connect the light to a timer you'll do best. In the summer they should get the longest light periods, and the shortest in winter, though the light period can be longer than natural daylight.
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