How to Quantize a drum loop, but later "Un-Quantize" the rest of the arrangement?

I have a drum track from a live drummer which has its tempo ebb and flow naturally, and I want to write music in Cubase to go with it.

I know how to quantize the audio to fit the metronome in Cubase… but what I want to do is, do that, write my arrangement, but then “un-quantize” my arrangement such that it fits the drummer’s original drum track. Is this possible?

In other words, the quantization I want to apply to the rest of my arrangement in the end is exactly the opposite of the quantization I’d originally be doing to the drum track.

(I should mention that I am aware that it’s possible to create a tempo map of a drum track- but I tried this out and found it to be too difficult to work with in practice for a number of reasons, so I’d really like to do something like what I’m describing above

Yes, that is possible. You need to create a tempo map, thus telling Cubase which bar or beat should happen at which sample of the audio event.

I think this is the point where we might be of help to you. Please feel free to describe in detail what you did and which challenges you encountered.

This video from Dom explains how to create and write a tempo definition into an Audio file. Once that is done, you can do exactly the kinds of manipulations you are after.