I used to use Outlook Web and might go back. The native client is a nightmare but does some useful things the web one doesn’t.
Docker has been a huge boon to my dev efforts, usually much lighter weight than an old school VM, so I’m no hater but if I had to work on battery more often I’d probably do the same thing, spend a few bucks on AWS and only work online but with better battery life.
All that still won’t get me more than 6h though.... :-(
Is Mail.app, or some other mail client, an option for you?