Using separate flows makes sense if you compose in dorico to picture as the OP does, when you are done composing you can copy/paste everything into one flow.
This is not what you are doing as the composition is done already and you only want to produce a professional score with dorico.
I haven’t been diving too deep into tempo operations within dorico, so these are just guesses:
For a DAW or a scoring app there is no such thing as a gradual tempo change. Behind the scenes they are a series of static tempo events. I would assume that your DAW exports static tempo markings as text, and the actual tempo change (the midi message) will be exported separately on the tempo track.
Your gradual tempo changes however probably are programmed very detailed to exactly match the picture.
Dorico probably interprets tempo text correctly, but how should it interpret a ritardando exactly in the way it was programmed in the midifile? The single tempo changes that make up the ritardando mus be kept as they are.
Would Dorico put in “rit.” as tempo text, it would apply its own gradual tempocurve which then would interact weirdly with the tempo events from the import, so it is probably a good idea to not try to interpret them.
Isn’t there an option in Dorico to simply ignore gradual tempo instructions either individually in the properties panel or as a global setting?
Alternatively, can you create a new symbol for gradual change that does not affect tempo?
Or maybe create a text that contains a hidden character (like a zero-width-space) so it reads “rit.” but won’t be detected by Dorico.
(I actually put one ot them between r and it. - maybe try to copy it into Dorico).