The H7 does not switch voltages internally, this is done with an external transceiver, if needed to hit some of the extended Ultra modes.
One can certainly get to rates in excess of 50 MHz with out switching voltages, and most microSD cards as I understand the specs are suppose to support >= 50 MHz operation.
If you run the H7 at voltages below the SD Card specification, you'll need to ensure the cards get the levels they expect.
The feedback clock is there to understand the inherent signal delays on the out-bound and in-bound journey, and so the STM32 has a reference edge for the returning data.
One can do a simple 6-wire + power/ground interface at 3.3V volts.
Even with a 26 MHz wire clock, I can get 6-7 MBps Write and 8 MBps Read from 4-bit eMMC devices, higher in 8-bit and SDR52 modes, and DDR ones.
Not sure what the plans are for the tranceivers, or the longevity of things that would replace it. I'd probably look to doing it with discreet ICs (modern 74 series), but not my circus.
