Athlete
Interim does mean in the meantime and by its meaning is not set in stone. I am not expert, but can't see any harm in coming to an arrangement that puts you on a temporary but stable financial footing. However if, for instance you could not manage, you could re-negotiate. You could poiint this out to your stbx.
You sound very sensible to me but remember to factor in winter fuel bills that are coming up. Items that are not monthly, such as holidays, birthday treats, servicing the car etc. What I did was add all these up and dvided by 12 to make monthly payment added to the normal everyday expenses. I know this may be stating the obvious but I would rather do that than not say anything
The fact your stbx is willing to pay and realising that is a useful starting point.
Best wishes
Shelia X