There are a lot of similarities between the 800D and the 80D. The 800D has a lot of 80D technology as the 800D was released after the 80D - but the 80D does have a slightly higher FPS, bigger body and has a higher shutter speed of 1/8000. The 80D would be a "modest" improvement, plus it claims weather sealing, whilst the 800D does not.
I am not much of an action shooter but I perhaps you would find a used 7D mkII to be "much better" than either 800D/80D with 10FPS and excellent sealing as a stills camera. And yes the 800D would still remaim great for video as a second camera.
Personally I think that the current 800D/77D/80D are great kit overall with what they can do and what they offer for the prices.
I think if you have mastered the 800D you won't have much issue with the 80D and probably some learning with the 7D mkII.
Canon 80D, 700D & G7 X; EF-S 10-18/18-135 STM, EF-S 18-135 IS USM, 50 F1.4, 100 F2.8L Macro, 16-35 F4L, 70-200 F4L IS; 100-400 L II, Speedlite 430EX II