I like this lineup, but would put Seshat on the left wing so her debuff fires early
Seshat - Joon - Richard - Mitsuko - Elkanen OR
Seshat - Joon - Richard - Elkanen Mitsuko
the second one is true to the rule of thumb of putting the tank’s weak color on the flank of the tank
Atamos is about as much a true tank as Richard, and while the latter is generally viewed more favorably, there are a lot of emblems on Atamos that may make him a contender for tank. If so
Seshat - Joon - Atamos - Mitsuko - Elkanen
One more possibility would be to put Frida at tank. Again, not a canonical tank, but she also has the debuff special, which is nice. She lack’s Richards emblems, though. Worse, she’s also a Paladin so she’ll compete for them.
We’re ruling out Azlar as a tank both because he is slow and because Mitsuko is so nasty against blue, right? With him as a tank, you could get Vela into the lineup.
In the end, I think @Chadmo is right, Richard may be your best tank at the moment.