Yeah I wanted a 10mm steel frame, and took note of those stainless steel frames. Unfortunately the race frames were for the smaller framed guns. They didn't offer them for the 10mm models when I looked.
A steel framed G29 would probably be better than a G20 version.
Sad to see they went out of business. They were still up and running just a year ago.
And as noted the Glock 21 is a 20 frame. It is the slides that are different (21 slide is lighter and less robust for the less demanding .45 ACP.)
So you should be able to toss your 20 slide on a 21 frame. And .45 ACP models tend to have more aftermarket support being a larger market, so you are more likely to find something for the G21.
Similarly the G30 has more aftermarket support than the G29, but is also the same frame and so many frame specific G30 parts will work for the G29.