Thanks for all the nice and useful suggestions and pointers! While all these are really practical and good suggestions (and I agree that Dorico is working correctly regarding its treatment of multi-bar rests), I just want to present my argument for consideration of a special ‘Tacit’ mode.
What if, as in my case, there is no need for the player to care about the empty bars (because he can hear them and he knows when his part will come)? Or if there is a conductor, he can inform the player to get ready? So if the first 18 bars can be condensed into a single multi-bar rest, then this will save a lot of space on the score. This is because (I’m presenting an argument here for a TACIT feature) the information created (the multiple changes of tempo) by Dorico in those rest bars provide no help to this particular player at all (he doesn’t need to know), and what he wants is a more concise score.