You won’t be able to get the very latest GPUs, and will be heavily limited by the performance of the CPU in these machines. I believe RX 580 is the newest you can get on an MXM card, and you’ll be almost entirely limited to AMD cards unless you’ll be running Windows on the machine. It will need to be flashed with modified firmware to see the boot options menu. Much better off spending your money on a newer machine. If you’re dead set on keeping it, there’s a wealth of info here https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/ on what to do.