Hi, if I install a standard non-flashed Metal-capable GPU in my Mac Pro 5.1, I won't be able to see the boot screen: does that affect the Recovery Menu, too?
Depends if the GPU is supported by Recovery. Recovery has drivers for AMD GPUs [Polaris (10.13), Polaris and VEGA (10.14), Polaris/VEGA/NAVI (10.15)] and the display will work after the GPU drivers are loaded - this takes around 3 minutes.
NVIDIA cards only work with Recovery if the GPU is a Kepler one, GT 6xx and 7xx cards. Maxwell and Pascal don't work with Recovery.
GTX 750 and 750Ti are Maxwell cards, not Kepler, even if starts with 7xx, while GT 750M is a Kepler card and used on late 2013/mid-2014 MacBooks Pro.