The advice seems to be that the best card I can fit in the SFF case without power the unit from outside the case is the Gt1030, however there was a model of GTX 750ti that was released in a single slot version and what looks like a low profile (although it isn't advertised as LP, this is the GTX 750ti model released by Galax in 2014. it's x16 slot is right next to the PSU, on other models there would be an extra x4 slot, thus the card must also be single slot.
Its two main limitations: (1.) it requires low profile graphics card, and (2.) it has only one PCI-E slot available, i.e. I am trying to find the best GPU possible to fit in a dell OptiPlex 3020 SFF, I have done a lot of research on this (read everything I could find online including the threads from here on the 3020) - the PC is limited in what graphics cards can fit into it.
