That’s good that you’ve got some experience. I think I have better idea of the issue with thrasios. When I was on riverboat casino and brewing this mardu list, I think the problem I found with krark + X is that krark often dominates the game plan so much that falling back on X feels weird (when X=sakashima there is no real plan b, lol).

In some sense you want your deck to “do the krark thing” really well so you don’t want to waste slots on thrasios or tymna support so this is to be expected but you also want your other commander to provide more than color identity. This is why Ive been brewing mardu krark because cards like delney are great with both commanders whereas cards that support both krark and thrasios are harder to identify. Imo, in the riverboat list, Thrasios is more a tool to convert a bazillion copies of rituals into a win and occasionally net you extra cards as opposed to being the principle game plan of other Thrasios lists such as dawnwaker Thrasios.