I don’t think it’s a matter of the supply pins. When I have the blue on-board LED off and the display connected, drawing a substantial current to power the backlight, and I read the SPI flash in a loop, this is the waveform I get on the clock line. Note that it reaches 0.5V within the 50ns on time, corresponding to the default 10Mhz setting of the SPI flash in the mainline kernel.
If I turn the backlight off, I get no change whatsoever; it still reaches about 0.5V.
If, on the other hand, I turn on the blue status LED, which gets powered from a GPIO pin, then the waveform is similar, but now reaches only 300mv or so:
For reference, here are the waveforms with display and DAC disconnected, i.e. with only the Pi 4B. With the blue status LED off, the waveform starts to resemeble a square wave, at least in that it reaches 3V, a bit before dropping back to zero:
If I turn the blue LED on, I get a 50% duty sawtooth wave again, peaking a bit below 3V:
I find it puzzling that the SPI signals are pretty close to unusable at the 10MHz default setting for the SPI flash. Perhaps something is wrong with my board (which is version 1.4), although I would find that unlikely. If you could verify whether you get the same behavior, that would be very helpful.