Holy shit I did it.
Beat the secret Insanely Hard Mode.
That was just... ludicrously bullshit hard, occasionally even unfair during the final bossfight (mostly thanks to Azure), but hey, bragging rights!
Protip for anyone else who wants to try this: Regeneration is your best friend. Even though you only start to get even a single Stamina per turn at 3 stacks, and don't get an extra point per turn until 5 and 8 stacks... that is enough. And the Health regeneration is actually enough to let you survive a single Sigma Attack, despite it doing as much damage as your default max health on this mode! And even 3 Stamina per turn can let you keep Stunlocking on opponent every turn for quite a long while (which you will need to do a lot, again mainly thanks to Azure).
But uhh... the reward feels a little... lackluster? A mere 5 bonus commands, plus some extra thing that can only be accessed externally, via emailing a secret phrase to the author. I love most of what this game does, and am at least willing to tolerate the parts I don't love, but... external rewards, especially ones that would get my email involved, are kinda... how about no.