Some of my thoughts:
1. This is an amazing idea!
2. I love that it is browser-based so can work everywhere. Native app would let you integrate more tightly (such as becoming a "keyboard" on the system), but that probably means "a mac app" which doesn't do me any good on Linux. If you could keep the bulk of it in cross-platform tech and just do the small integration part with native code, I think supporting at least "the big three" is doable. I bet if you provided a good API, somebody in the open source world would even do the work for you, on Linux at least.
3. Would really prefer being able to sign up with my email, and not having to log in with a third party account.
4. Online-only access is definitely fine for now, but to stay competitive in the future I would keep an eye toward being able to run inference locally so you don't have to be online to use it. This would also be a way for you to reduce costs and offer a cheaper version. If I were you, my long-term goal would be for this to be used by everybody (though that's years down the road). Local inference does complicate monetization, but that can be figured out.
5. For me to really use this enough to pay out every month, it needs to be relatively easy for me to get the output into whatever app I'm using, whether that is Chrome, Slack, Gmail, Google Docs, Vim, Gedit, or anything else. This is undoubtedly related to item 2 above, but I figured it warranted it's own mention as there may be solutions besides browser-based vs. native.
6. You're gonna have competitors hot on your heels, if they aren't already. Google in particular with GBoard on Android could be absolutely killer. Since it is Android-only, I don't think it's a major competitor now, but if they broadened it absolutely could be.
7. Do you have an exit strategy in mind already? Would you be willing to share anything on that? (I ask because it's relevant because your product could easily become part of my standard workflow, and I'm very conservative about becoming dependent on proprietary products, especially from startups). Please do not go native-only and only release a Mac app. At a minimum, please maintain the web-based version. And please for the love of all that is holy, don't sell to/get acquired by Apple! I want and need your product, and I don't and won't switch platforms (Fedora Linux currently) to get it.
Really amazing idea and great work! It is rare that I see products that I think could actually "change the world" but this one has some potential by changing the way we interact with our computers!