If you find deault hard then you'll hate manual
Because there's no assistance from the CPU, naturally you have a lot more freedom with passes. You can do a lot you can't do on assisted, e.g. certain types of shots and through balls, crosses, etc. It's harder obviously, but once you play it a lot you'll get used to it.
I never really played semi so I'm not too sure. Although semi is more assisted than manual, I think it just requires you to choose the power.
If you were gonna try manual I'd advise you put lobbed balls and through balls on manual first before anything. I don't know notice any difference on lobbed balls and through balls are much better. I use them as a replacement for long ground passes as they travel much faster instead of bobbling. Unlike assisted through ball you have to power it up, but you get used to that.