GPU suggestions for mac

looking to upgrade the GPU on my macbook pro. TD can get a little laggy sometimes and I’m assuming it’s my GPU. I’m extremely new to this world so maybe you have other suggestions. Never put an emphasis on graphics before, coming from a music only background.

this is what I currently have:

Radeon Pro 450 2 GB
Intel HD Graphics 530 1536 MB

side note - should I be running TD on an external hard drive? or will that not make a difference in performance?