That aside, you seem to be conflating "free as in speech" (FOSS) with "free as in beer" (freeware, or how most people treat WinRAR) in some parts of that writeup. Ardour is the usual example of software that's the former without being the latter (you can download the source code for free if you have the know-how to compile it yourself, but a donation is required to download the pre-built direclty-installable binary).
It's also problematic for educational institutions even if they happen to be tolerant of a little bit of colorful language here and there. By charging $30 a seat for the "educational" version and wanting to forbid educational institutions from distributing the "free" version, you're punishing the Goonies-tolerant organizations for Goonies-intolerance. Not to mention the need to define what "educational institution" means.
Being concerned about preventing others from making money off this ("If we struggle to make it free, you can’t be paid") seems to also run counter to the stated reason for making it free in the first place ("We will make money from creating great stories, not selling overpriced niche software.").
There's also the problem of longevity. If Wonder Unit, Inc. goes belly-up, then its non-FOSS storyboard editor does, too. An actually-FOSS license means there's at least some chance of the broader FOSS community keeping it alive, if only so that they can say "well hang on, Linux has awesome creative/multimedia tools, too - and we don't even have to pretend GIMP is one of them anymore".
All that said, the MIT license is probably not the right fit for Storyboarder itself (but is certainly a good fit for the underlying libraries; same with the ISC license) based on your criteria. A strict copyleft license - like the GNU GPL - might actually be more up your alley, since it would force anyone hoping to make a buck off your hard work to release their source code and allow others (you included) to do the same and make a buck off their hard work. If they don't comply, sue 'em.
Of course, even that one educator's little stunt of cloning your repo and removing all references to Wonder Unit would run afoul of both the MIT and ISC licenses, so you'd still be covered if you took the legal approach to going medieval on his Goonies-intolerant ass.
All in all, it doesn't really matter to me (as a Linux-using end user) what license you opt to use for Storyboarder (I use plenty of non-free software day-to-day), as long as there actually is a license so that I know whether or not I'm legally allowed to even use the software (let alone redistribute it, modify it, package it up for a Linux distro's package manager/repositories, etc.).
Even with all that aside, though, Storyboarder looks like an amazing tool, and the fact that it works on something that's not running Windows or macOS is a pretty damn big deal. Kudos, and thanks for your hard work. Whatever the license ends up being, it'll be a valuable contribution in that particular niche of "storyboarders who want to use Linux".
That aside, you seem to be conflating "free as in speech" (FOSS) with "free as in beer" (freeware, or how most people treat WinRAR) in some parts of that writeup. Ardour is the usual example of software that's the former without being the latter (you can download the source code for free if you have the know-how to compile it yourself, but a donation is required to download the pre-built direclty-installable binary).
It's also problematic for educational institutions even if they happen to be tolerant of a little bit of colorful language here and there. By charging $30 a seat for the "educational" version and wanting to forbid educational institutions from distributing the "free" version, you're punishing the Goonies-tolerant organizations for Goonies-intolerance. Not to mention the need to define what "educational institution" means.
Being concerned about preventing others from making money off this ("If we struggle to make it free, you can’t be paid") seems to also run counter to the stated reason for making it free in the first place ("We will make money from creating great stories, not selling overpriced niche software.").
There's also the problem of longevity. If Wonder Unit, Inc. goes belly-up, then its non-FOSS storyboard editor does, too. An actually-FOSS license means there's at least some chance of the broader FOSS community keeping it alive, if only so that they can say "well hang on, Linux has awesome creative/multimedia tools, too - and we don't even have to pretend GIMP is one of them anymore".
All that said, the MIT license is probably not the right fit for Storyboarder itself (but is certainly a good fit for the underlying libraries; same with the ISC license) based on your criteria. A strict copyleft license - like the GNU GPL - might actually be more up your alley, since it would force anyone hoping to make a buck off your hard work to release their source code and allow others (you included) to do the same and make a buck off their hard work. If they don't comply, sue 'em.
Of course, even that one educator's little stunt of cloning your repo and removing all references to Wonder Unit would run afoul of both the MIT and ISC licenses, so you'd still be covered if you took the legal approach to going medieval on his Goonies-intolerant ass.
All in all, it doesn't really matter to me (as a Linux-using end user) what license you opt to use for Storyboarder (I use plenty of non-free software day-to-day), as long as there actually is a license so that I know whether or not I'm legally allowed to even use the software (let alone redistribute it, modify it, package it up for a Linux distro's package manager/repositories, etc.).
Even with all that aside, though, Storyboarder looks like an amazing tool, and the fact that it works on something that's not running Windows or macOS is a pretty damn big deal. Kudos, and thanks for your hard work. Whatever the license ends up being, it'll be a valuable contribution in that particular niche of "storyboarders who want to use Linux".