Conceptually this is a bit too similar to Transcendent Myth IMO.

I was gonna say "Whoa; it proclaims everything and massively powers up everything else"

And then I saw the rarity and casting cost; and that went from "Eeep" to "Ah, intentional use; very cool". Although one problem with proclaim in general, having got to 7 mana with lots of cards in hand seems very hard to do.

Aye, that's why Masumaro, First to Live is mediocre at best. You can think of it as a puzzle though. To get most of the card you could spend mana on your earlier turns to ramp instead of proclaiming your cards. Forgive and Forget also should help with having too few cards in hand by the time you can - or after - you have cast this.

I'm not a fan - it definitely feels like it fills the same archetype of card (not draft archetype) that Transcendent Myth does, like you mentioned. It might be unintuitive that you can proclaim stuff in your hand without it having a proclaim effect, too. Does proclaim have any other win conditions other than this one? Might be better to use this slot as a powerful enabler rather than a payoff.

Win-cons? Not really though I don't feel there necessary has to be bombs with this mechanic slapped on them. Valinorian Mûmak maybe?

For enablers there's Plead in Whispers, Elf-Sage of Valinor, Vairë the Weaver, and Moon Guide. Royal Messenger also likes proclaiming but it's negligible. Plead in Whispers I might change to something else since it can blur what "proclaiming" actually means... I think I was supposed to change it awhile go along with Elf-Sage of Valinor since it had a similar trick before but I guess I just forgot to do that.

This is also an enabler as well: It started out from the idea of "proclaim for free" or "proclaim all the cards in your hand" but I wanted to wrap it into a sensible package that works on its own. Okay, maybe it's more of a payoff since it costs so much so yeah. Without the last ability, I could see this costing maybe, but IMO it would be a slightly lesser design.