An interim GRC does not legally change someone's sex. It is granted where a person meets all the requirements for obtaining a full GRC except that the applicant is married/in a civil partnership and their spouse/CP does not consent to the marriage/CP being converted to a same sex/opposite sex marriage/CP (as appropriate).
The interim GRC is basically a certificate that says, 'we will allow you to legally change your sex when your marriage/CP has ended and either party can use this certificate to do that.' That's all it does.
The most important principle here is that a marriage or CP is a legal contract between two people and the terms of a contract should not be changed without the agreement of both parties.
Where the marriage ends amicably, the obvious solution would be to separate for the two years 'living as' the preferred gender (a requirement for getting a GRC) and then use the ground of two years' separation with spousal consent to obtain a divorce. That's also plenty of time to come to an amicable agreement about the division of assets so the divorce should be quick and easy.
It's normal to sort out the division of assets before a marriage is finally ended. I'm no expert but I imagine it could create all sorts of problems to do it the other way round. What happens with housing rights? These can be different depending on whether you are married or not. What would happen if one party died inbetween the marriage ending and the division of assets?
I can see no good reason for turning all this on its head simply for the 'principle' of being able to obtain an all-but-obsolete cerificate a little earlier.