at FRA you get punted and have to re-enter through security to
continue your journey. I haven't passed through FRA, inbound, in the
last 6 or 7 years so maybe that has changed and you can now stay
I've flown into FRA 7 times in 22 years, once in the last 5. Four times were non-stop flights from Denver; three times I had US connections (DTW twice, ORD once). FRA always been my final destination and my Schengen entry, so I've never had to worry about time for immigration.
I once flew to Stuttgart (STR). STR was my final destination and point of Schengen entry. Had I flow FRA to STR, I would have had to go through immigration in FRA and worry about connection times. That's why I connected in ATL on this country on my way to Stuttgart.
I've really tried to figure out the logistics of transferring at FRA, mostly by looking at You Tube videos made by people doing just that. I've also been airside (outside of Schengen) at FRA three times in the last 10 years, twice in Z, once in B. As near as I can tell, you can move between Z, B, C, & D/E and remain outside of Schengen, as long as you use the SkyLine train. However, to use the tunnel between A and out-of-Schengen B, you have to go through immigration at each end.
There seems to be three "sides" at FRA. There is the public side (Landside) that you can get to without going through security. You can use the SkyLine from Landside, but you have to go through security to get into any of the concourse. Then there are the gates in the concourses that are "airside", that is inside security but outside Schengen, and then there is what is called airside on the SkyLine, which is not inside Schengen (i.e., you do not have to go through immigration to use that side. However, to go from an airside car on the SkyLine to the non-Schengen gates of a concourse, I believe you still have to go through security.
The SkyLine has cars that are "landside", that is they go between the outside security areas of B and D/E, and it has "airside cars that go between the outside Schengen areas of B and D/E and the only-non-Schengen gates in the C and Z concourses. So it should be possible to make outside Schengen changes at FRA using any concourse.