The speed does not seem a problem to me.
You said it yourself.
... and probably a diesel engine system, the ships would not be able to keep up the speed of other Kriegsmarine surface units, so their military used for the Kriegsmarine was very doubtful.
In KM'46 the ships are useful, due to their speed of 32 kn.
How did you manage 32kts when the actual engineers couldn't?
I repeated the german-navy.de-text, a text from a real projected (ok, never really planned) ship. But this is KM'46, an AU. Here the CVE has got a third screw and larger engines and an improved hull, so it CAN reach 32 kn. It's likely to get caught in the 'reality vs. AU'-trap, I know. We don't leave the space of real physics in AU, we just create new or bend some existing timelines. In my opinion, an for that speed optimized hull and enough engine power would be enough. Ok now, friend?