FYI, Bitwarden doesn’t submit the login form when you autofill “on page load” — user action is still required to confirm the intent to log in. Furthermore, most websites have a dedicated login page, that you won’t reach accidentally by just browsing the site (it requires you to click a “Sign In” link of some sort); it is relatively straightforward to prevent Bitwarden from autofilling unless you have actually loaded the site’s login form.
Again as an FYI, in case you weren’t aware, there are several solutions available that allow you to map keyboard shortcuts to mouse actions.
Your frustration is understandable, but as I have no affiliation with Bitwarden (other than as a customer), it would be more effective to voice your concerns to Customer Support or by posting in the UI feedback thread (which I see that you have already done).
I have many concerns of my own about the redesigned UI, but autofilling is not one of them. I did propose a suggestion for how to improve the autofill button in the feedback thread; perhaps something like what I suggested would help users who use the extension like you do.