Therefore, you need to add your sink allowance into the MDA anyway
Interesting. Not sure what the convention is in Europe, but we Aussies are trained to not 'sink' below an MDA, especially in a circling procedure. Had it hammered into my skull in initial training that you can sink below an RA/DH but
not an MDA.
Use a rule of thumb such as 1/10th of the RoD to determine the height above MDA where you commence the level off so you don't sink through it. Tolerance in maintaining MDA in our checks is +100FT, -0FT.
Having said that there if it is a database runway approach with a vertical profile, commencing a level off prior to MDA can be counterproductive in the infrequent occasions that the Wx is right at the MDA. you won't bust the MDA but you will immediately put yourself high on approach profile, as you allude to, and very likely not recover the slope within stable approach criteria.
However our friend with the question is flying something with 9 to 19 seats, not 119, and probably jotting down his own vertical profiles on a notepad instead of following a VDI. In which case he might be constructing a profile that gets him to the MDA nicely before the 3deg slope.