All iMac GPUs are custom designed boards by Apple around
various graphics chips. As such, they are specifically designed
for each iMac model.
So, the only option you have is to replace what you have with a version
built for that iMac at that time that has better performance. If you purchased
the best GPU available for that iMac at that time, it is the best that you
can do. There are only a few places that carry parts for Macs that old
and could be tough to find.
Probably not what you want to hear, but is a fact of life with an iMac and
most any "all-in-one".