As long as boards are supported by kernel and U-Boot not much work should be needed, hopefully. Just some kernel configuration and adjusting ALSA configuration for analog audio (HDMI audio is not supported AFAIK).
So, if you don't mind figuring where your MK808 board is, I would go with it. Just to be sure, that's MK808C, right? A20 should have slightly faster display engine afaik.
P.S.: I just noticed that required kernel patches are in 5.1. Oh well, picking few patches shouldn't be too hard.