How can I export a single instrument as a midi file, in a project with multiple instruments?
I don’t think that the MIDI export procedure currently has any built-in ability to select or deselect what gets exported. However, you could try suppressing playback of the instruments you don’t want included in the export - select all the notes in an instrument and do Properties > Common > Suppress playback. You can do all adjacent staves together - select the first note in all those staves and do Edit > Select to End of Flow. If the wanted instrument uses the top or bottom staff in a score, you can do all the others in one go. If the instrument uses one of the inner staves, you will need to select all those above it and do them, then select all those below it and do them.
After typing the above, I have noticed that you are using Dorico for iPad. Without actually knowing, I would assume that the procedure is the same, but possibly a little more cumbersome with the selecting unless you are using a mouse.