Is it possible......
To use a normal angle grinder to cut off the centering ring? How hard is the metal?

Can it be done on the car?

ouch...
why...

couldn't you have your wheels cut at a machine shop to fit onto a ringed hub?

A much better solution is to have the back side of your wheels machined to accept the ring, as it's easier to R&R wheels/tires & change flats.

Machine like this pic....
Click to view attachment

.

Or get the proper wheels.

It's easier to do it right.