Since starting into Dorico way back when, I’ve always found the Transpose function to be really clunky.
I’m working on creating a method book for my students and transposing the same content into every key.
While the ordering of the variables makes sense when you’re reading from top-to-bottom (Quality, followed by Interval), the way Dorico filters the options is backwards.
The Quality box is first, then the Interval box is second - that’s how we verbally describe intervals (Major Second, Perfect Fourth).
But if I want to transpose a line of music by a minor third, I cannot select “minor” first. I have to change the interval first, and then go “backwards” to change the quality.
Since workflows usually got from top-down or left-to-right, as a user experience, I feel like I’m bouncing all over the map when I want to change intervals.
I think it might make sense - and a streamlined workflow - to only have one drop-down to pick from, but with a sub-menu structure.
The user would first pick the base interval (seconds, thirds, fourths), and then that would extend out to another level to pick the quality.
Maybe it’s just me, but it feels like the dialog box is not sorted out well currently.
Thoughts?
Thanks for reading. 
Try abandoning the transpose dialog and just use the shift-i popover instead.
To transpose by a minor 3rd use shift-i t3m, for a major 3rd use shift-i t3M, for diminished 5th shift-i t5dim etc.
Fortunately the note tools popover also accepts quality-before-interval, like tM3 (the grammar the OP was hoping for).
This issue has been brought up and discussed before. See this thread, for example.
The Shift-i popover is very usefull, but I just found out that it acts in a different way than the dialog box: it does not change the key signature, you have to do that afterwards by yourself, while the dialog takes also care of that!
There is, of course the SHIFT + K popover to change key signature if that route is easier for you.
(Really, although it may seem opposite to how many folks think, how difficult is it, really, to navigate the Transpose dialogue?)
Actually I have no trouble with the Transpose dialog any more because I always use the calculator on the right side. When I want e.g. a minor 3rd up, I set it to D and F, etc. I hit Apply and confirm that it understands the interval I intended, and then I don’t have to bother with the syntax on the left.
But that too would be “the wrong way round” from how we verbally describe intervals. I’d choose “third”, then “minor”.
I think something like this would probably minimize the problem.
For me, the “Quality” box is the problem… it’s meaningless until the “Interval” box is changed first if you’re switching between perfect and imperfect intervals.
Ryan J Williams,
Composer, Arranger, Educator, Clinician
White Mage Music • www.WhiteMageMusic.com
Choosing the interval first before the type prevents one from hastily choosing a mismatch such as a “major octave.” It is no more difficult to choose the lower pull-down first than to choose the upper first.
Were Dorico to let one choose major/minor first, undoubtedly someone would complain that the interval box only let them choose seconds, thirds, sixths, and sevenths when they wanted a fourth.
As long as I can get the program to do what I need, I prefer just to learn the program rather than getting too upset over offended sensibilities.