If anybody is interested, I've been writing a Gmail-focused email client for macOS called Mimestream. It combines native macOS UI with Gmail-specific features such as categorized inboxes, colored labels, and aliases (made possible by the use of the Gmail API instead of IMAP).

I just opened up a public beta, and you can request immediate access on the website at https://mimestream.com. It's free during the beta period - I only ask for feedback: feedback@mimestream.com.

Unlike some other 3rd party email clients, it makes direct connections from your Mac to Gmail and stores data/credentials on your Mac. There's no intermediary servers.