I agree it is shorter, though how often do you or others in general use H3 and H4? A hashtag button might come in handy in other situations too. Maybe a menu to enable/disable certain buttons and to change the order of the buttons could be an idea?
Some thoughts on the submenu. You could center it one row above the initial button, maybe with some fixed left and right barriers? This would give the advantage (if added) that in case of a false click I don't have to touch the x but could simply click on another button and the submenu would disappear. I don't think it would be good to make it disappear upon a click to the text as there you might have to place the cursor to apply a style.
I'm impressed by the immediate reaction time in your demo. Your screen size seems pretty large to me. Did you also try it with smaller screens and do you consider to scale the toolbar somehow? On my Iphone SE the normal keyboard already takes more than half of the space in joplin. I am left with ~400px of the body. Maybe it is possible to integrate the "done" button or a keyboard down button into your toolbar and remove the native toolbar, as this would give it some space and I assume you're not going for native ios integration anyway.
If you use more than a bare limit of buttons I think you'd have to implement some sliding functionality like MDNotes has. I looked up some display resolutions:
Apple's display resolutions start at 640x1136 px. Is that roughly 1/2 of your current screen resolution? I checked some of the most sold android devices and they seem to start ~720px in width and the android developer site has some resolution spec to support different screen sizes.