Low Profile, Single Slot RX 560 @ Amazon.com
This is kind of like the Unicorn of RX 560s. Didn't think such a card even existed, but here it is. Made by Visiontek.
LP RX560 at Amazon
That's the good news, the bad news is the price they're asking. Over $180 USD. On the plus side, these 560s will give you many more years of compatibility with future macOS versions. The NVS 510 or Quadro K600 may possibly lose support in the next version of macOS, 10.16 coming out this Fall. If you want to or have to, stay on the very latest macOS version for whatever reason, this might make sense for you. Important for those that develop iOS and other apps with Xcode.
You can install one LP RX 560 into your Optiplex and only use up one PCIe slot. Has a small fan and a good sized metal heatsink around it. All four outputs are mini-DP. It's similar to the NVS 510 by Nvidia which also. This does appear to come with a full height bracket for use in the Optiplex MTs. Buy new and you'll get all the extras pictured below. It is best to keep a straight mini-DP to DP connection to your monitor.
In the 7020/9020 SFF models it will be a tight fit next to the PSU. I haven't tested one of these yet. If anyone does, let us know how well it fits in the SFF x16 slot on the motherboard.
You can also put it in the x4 slot but you will lose some performance due to the reduced number of available lanes. If you want to run dual or triple 4K monitors, definitely put it in the blue x16 slot instead.