Disclosure: I work for an established (maybe?) competitor to this idea; I say maybe because it's really not clear what you do until I read most of this thread, nor how you do it (or what's planned vs. available).
First, Welcome! This is a huge arena and I think there's a million, nuanced areas for companies to tackle and really focus on amazing merchant experiences. THis is a very big (and growing) pie.
Second, I think you'd benefit from trying to increase the clarity of what you do and the value proposition, plus how it all works. For example, I'm not sure how you can provide broad-based api support for 3rd party developers without script injection or strong webhook support, but maybe you've got a new approach? I also admit to being skeptical when you mention a feature as surpassing platform rate limits; we have a continual pain point with shopify, query cost calculation and throttling as an example.
Third, we also use a platform-agnostic solution, but then build internally on this before exposing client-facing services. After several years we don't support nearly as many platforms as you; how do you stay on top of all these unilateral changes? Just shopify keeps us on our toes, and some of the ones you mention have little critical mass making them unattractive.
Finally, it would probably help adoption if you at least gave some indication of what your pricing is going to look like, even flat rate or %, or totally custom... You're asking people to build solutions on top of your product without any sort of idea of what the long term ramifcations look like. That's a hard sell.
Good luck with the venture!