I've tried Thunderbird, Kmail, and Apple's client, and maybe I just have too many emails, but these apps completely crumble under my inbox. I don't see any "shine" with these third-party clients. Mimestream, my favorite email client on macOS, "just works" because it uses the Gmail API. It seems like Fastmail made JMAP, but this doesn't seem widely supported.

Are there mail clients that actually support things like priority inbox and categorization that don't simply crumble for large inboxes?