Hard to give feedback because your design decisions and trade offs are not public and in your writing you give more of a business vibe than a designer, would've been nice to have the designers here instead of you. But you asked for it so here we go...
Good attempt so far. Frankly, something like it should've been "a mode" for iPadOS. Instead, unfortunately, you're app is restricted by its horrible limitations. There's no separation between content and features that can be applied to content. Take PDF just as an example. No matter how good your team is, the chances that it will create a better toolset than what Adobe already has created are slim. The second I decide to interact deeper with any of Muse's supported content types while in a board, I'm left to leave Muse and deal with an entire different app. Result? My flow is broken. This interaction is conceptually unnecessary. If the PDF is already open in Muse, why can't I bring all the great features from Adobe in and when I'm done I continue with my flow?
2. Your metaphor needs a lot of work. I won't go into many details but I'll highlight one of them. What's up with the grids? Why can't I arrange the content anyway I like? Flipped, 34 degrees, etc.
3. Infinite canvas - bring this in. How to deal with it spatially was solved decades ago.
4. This is related to the last two points... computers are a meta-medium and either you are stuck in the medium that existed before or you're not letting your designers that know this not shine. Stop thinking only in 2D!