Your MBP is still a remarkable powerful machine, with big display, dedicated graphics card, and a lot of desirable features. Plus, Apple products tend to keep good value over the years.
MBPs 13" play one class lower. (I also have a 13" which is enough for me, I don't do many graphics. And I always have an external monitor.)
Again, this depends how much you use. There should be a certain amount of free space on a SSD, between 10 and 20%. If at the moment, at your 512 GB SSD, you do not use more than, let's say, 150 GB, then 256 would be ok. Otherwise...
There are many people around who do not mind having a rather small internal SSD and plug in a big external one when space is needed. Personally, I prefer big internal space but everyone is different.