Brian Gaff wrote:

Save a non compressed master of this intermediate mix, then save your
new mp3. If changes are then required
go back to the uncompressed version and add to that, not the mp3.

That's essentially what I'm doing.

Or use Reaper and simply save the project and do the editing afresh, but
that software is really hard to grasp and not good for simple stuff.

I'm not familiar with Reaper and while I wouldn't exactly categorise what
I'm doing as simple, I'd prefer to stick to software that I'm familiar with.
But Reaper sounds interesting and I shall give it a look, thanks.