I was never into K9 Mail back in the days where it still had a Holo-ish UI, so I never really got used to it. But I decided to join here only to find myself looking at quite a lot of backslash. I agree that this design will not hold up in the long term, given the increase in screen sizes.
I don’t have answers for every single problem, but I think there might be a few solutions to make the new design more bearable for older users.
First, the account overview. I think that hamburger menus are starting to be out of date. I believe that a menu popping from the bottom would be best. At a first tap (or swipe up from bottom), it will appear and show a similar layout as the current hamburger menu, and it should allow to swipe left/right to see the different account icon, so they are at two taps of distance. Swipe up again, and you can see the current email folders and such.
The easier solution would be to just display the other mail accounts in the current side menu at the bottom, above the “Manage settings” function. The downside of this is that the mail folders would be further moved to the top, making them harder to reach and will have to be reworked again in the future.
Yet another way of working this out would be to move the whole top bar to the bottom (The search and write buttons are better there anyways, the 3 dot menu can be expanded too). And make accounts similar to the tabs button in such browser. You can tap it, and select the tab (account) you want, and you can also swipe left/right to switch accounts quick with a single swipe.
Whatever happens, I think everyone agrees that switching accounts is painful, and the main problem that needs to be solved, At the very least, setting custom icons for accounts instead of relying on contact information (I have all my accounts in the same contact, or no contact at all, so its hard to differentiate them)
Thanks so much for your hard work, I hope you guys manage to keep improving.