[toni-a]

From what I read the problem is not surely in the camera body.

You don't need extreme sharpness for portraits but I don't know how you can live with manual focus and portraits, you have a thin depth of field plus moving subject, not the ideal combo.

70-200f4 can be an acceptable performer but you have also the 60mm f2.8 macro that should do the job

I suggest you start shooting RAW and work on the lighting: reflectors, flashes, gray card for white balance, in portraits a white balance that is off and harsh shadows can be truly toxic.

Now about the sensor, my 50mm f1.4 shines on crop cameras for portraits while I don't like using it on 5D, however my 100mm macro is absolutely wonderful on my 5D while it is too long on crop cameras.

which pictures do I prefer 750D plus 50f1.4 or 5D plus 100mmf2.8 ?? really hard to say, both are great. my advice forget about the camera get a reflector, a flash, a grey card and start experimenting, you will discover  very wonderful side of photography you were missing.

For portraits priority is lighting them come all others, anyway photo=light graph=painting/drawing so photography is just about painting with the light