It's not that simple.

The last couple years, a buddy of mine did exactly what you proposed: He had massive GI problems, so he started watching what he ate, and… his problems became much worse.

Fast forward to a couple months ago, when he sees a doctor specializing in this stuff and she tells him to significantly reduce plant-based food for a while "because your gut biome isn't able to handle it right now. You should go eat a burger every now and then." So he's been following her advice and all of sudden he is without any GI issues whatsoever.

My buddy got lucky (found the right doctor, could eat his way out of it) but if your gut biome is seriously out of whack, I can definitely see the appeal of an FMT to kickstart your recovery.