Apple uses special Apple edition video cards, you can't just pick one off a shelf.
This is the shame of Apples iMac, the upgrade path is limited and super expensive if it exists at all. The new ones are even worse. I have a 2007 imac and it was way easier to work on than later models. If cost is the big issue you would probably be better to just do you light room on a PC. The requirements aren't that high, just unfortunately higher than what your Mac will do. Apple really doesn't care, they figure you should be buying a new mac every few years if you wan't to play in their ecosystem, they won't even sell you replacement parts for computers that are no longer supported, even if they had them. I use them everyday at work but just can't justify buying a new one for home, for exactly the reasons you are now experiencing. If you don't have a Windows pc, you could buy a very inexpensive refurbished machine and add a cheaper new technology card to it and it would work fine with Lightroom. If your using a creative cloud subscription for light room that will tranfer to the PC. Good luck!