Found this thread super late, when I was searching to see if there’s any news on the MailMaven front. Wrt to filing messages, which I heavily relied on Mail Act-On for, I came across a tip elsewhere which helps. In macOS Keyboard>App Shortcuts, if you set Show Help Menu to something (I use ^⌥⌘/), you can hit it when a message is selected, and then type part of the mailbox name you want to move the message to, and it brings up menu options containing that string, the first one being “Move to (whatever)”. You can set up Mail-specific keystrokes to move to popular mailboxes, or move to an arbitrary one by ^⌥⌘/ then partial mailbox name then down arrow then Return. It’s almost as good, and keeps me in Mail.app. I’d still try MailMaven, I think, but the known issues and bugs and feature request lists presently still look VERY LONG, and I don’t know that I can really wait for them to be done. I’m two OS upgrades behind on my laptop just so I can continue to use MailSuite and I’m pretty close to the end of my rope. This Help menu trick helps.
I’ve been trying to figure that out for AGES!!!