Not sure I follow you here. The internal graphics is part of the Intel CPU chip. There is no discrete GPU option internally.
An external GPU option is really not possible as the older Thunderbolt cases are no longer available for your series, you might find one used. It was a good idea! But it didn’t really work effectively if you are using the internal display as the bandwidth was not large enough.
As far as what GPU board, again the bandwidth of the connection won’t allow you to get the full performance of the GPU board.
The bottomline here is between age of your system and the limits of both its graphics and external I/O, I would recommend getting a much newer system which offers the graphical performance you are looking for. I would look for a used 14 or 16” MacBook Pro.
And if you still want more extreme graphics performance get the current Thunderbolt cases with the GPU board that meets your needs.
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