Yeah, it's really that simple. I have tried various applications as well and keep coming back to my custom script because when a new voice model drops on HuggingFace it becomes possible to customize it immediately - rather than wait for that application developer to support that new model.
Yes, the keyboard shortcuts are available, but I am unable to see browser navigation shortcuts like 'command + left arrow' to navigate to the previous page.
It should be JMAP because that's the primary protocol that Fastmail as a email hosting company (not a email client) operates on.
I don't think that's open source based on the blog post alone, and I don't think they have a reason to build a separate email client with advanced features. This app is likely just meant to serve the needs of the business clients that have asked for a separate app rather than using the website.
Now that you mention it, I remember that's how Fastmail works today. They use app passwords for non-Google-hosted email inboxes, and 'login with Google' for Google-hosted inboxes.
Do you mean to access Fastmail as a client for your Gmail-hosted inbox? How about using the fetch feature that moves all emails to your Fastmail inbox, and the 'send as' feature where you can send emails out from your Gmail SMTP via Fastmail. Those two features already exist.
On the same boat as everybody else that this is electron based and not a great experience. I think from a business perspective this is just meant to satisfy those customers that are asking for a standalone app.