Best MBP upgrade from 2011 MBP?

you have a quad core i7 in that thing, old as it is CPU power has not progressed like GPU power has, most of the dev in CPU appears to be battery life for portable devices.

If you added 16GB RAM and an SSD to that thing it would be like getting the computer out of the box from Apple but significantly faster in every respect.

If you go this route your RAM needs to be specific to that system and verified to work with that system

OWC "Macsales.com" and Crucial are two vendors who do sell RAM that will work with that mac. Avoid RAM with the same specs, voltage, latency and timings you find elsewhere, they may be cheaper but there is a good chance they wont work.

for the fraction of the cost you would be getting a great system, and the 2011 still supports High Seirra.

If you need a GPU later you can get an external one, while this is going to be expensive it is not going to be as much as a new mac and there a a number of GPU options available that would essentially blow the doors off any GPU optimized application in comparison with the top of the line MacBook Pro.

There's nothing really wrong with your current setup. In fact I rather like being able to swap out the RAM and the SATA drive. An SSD would be a nice improvement if you haven't already installed one. I believe yours may be the first generation where they used a SATA III (6 Gbit/sec) connection for the drive.