Hey! Decided to create an account because this mod is amazing. Seriously, weapons showing is lowkey amazing, but I LOVE the way you did unarmed. It feels so... well, comic book superhuman. I have a giant floating eyeball spawned from the corpse of insanity... and I fight it by ragingly charging at it with my fist. It is dynamic, and I think mostly reflects how to make melee work in a game like this.
That said, I echo other sentiments; fists get OP. Except to say in "hardmode", I think, pins it too late. For vanilla bosses, at least, it does not take long, mostly with content just from this mod, to break boss fights, in Expert. First time I got to hardmode with this mod on, with no HM armor or weapons, only the demon fist, I got all 3 mech bosses to spawn pretty early. The Twins at least could hurt me, mostly cuz I wasn't really prepared to fight them and had not yet decided best way to deal with them as unarmed (IME, reverse the order of which eye dies first from what seems to be common practice.) But Skeletron Prime died far more easily than he should have... and I actually just broke the Destroyer fight.
The thing is, it isn't even the damage. I mean, the fists hurt quick, but they still have to connect. Rather, the synergy of the I-frames, attack speed, and lifesteal/regen on hit effects mean that it is way too easy to build a character who, simply, sticks on a boss like a fly to honey cuz that keeps him going. As fun as it is in some ways, it means I break boss mechanics, have no use for boss arena builds, and relish what is supposed to be a commonly avoided boss tactic, trying to clip me. Like I said, for Twins, I found kill Retinazor first better, cuz Spazmatism is too stupid to stop getting in my face to blast flames... which I just punch through.
I think the way you set up the sashes suggest what you had in mind: wait for a good moment, dive in, get some shots, learn when to disengage before you get hurt and you can heal up any residual damage. But again, ESPECIALLY with Frenzy Heart, the other option is Just. Keep. Punching. And not even dives or uppercuts or charges or whatnot, but just simple punches, cuz that keeps the I-frame, stick to target chain going. I can't say for sure the best solution, but one bruteforce method that comes to mind: set a timer on the I-frames. It is only cuz I am likely to get multiple hits in before I next get touched, which gives me plenty of chances to regain health and just procs Frenzy anyways, that this works. If you had, like, 5 seconds or so before you HAD to stop punching, it would at least give time for the boss to actually, ya know, do its thing. Include a slighter cooldown timer to taste. Again, I think the mechanic is mostly right, it just needs to somehow make it that you HAVE to let up and spend some time still playing the boss as intended. But right now, it is too easy to just say "nah, imma punch moar."
EDIT: That style slash looks amazing.