I use Thunderbird because I get roughly the same user experience across all platforms. For linux-only I’ve heard a lot of users love evolution, though I’ve never daily-driven it myself. I can’t even answer whether it meets your requirements, but wouldn’t take long to install it and find out for yourself.

Proxmox Server > VMware Server

I’m curious what sets this apart from the existing JuiceFS approach? JuiceFS is a commercial product, but with a foss community edition. It doesn’t appear as a block device, but it does allow you superimpose an ext4-like filesystem complete with metadata etc. over S3 block storage (and others I believe). I’ve been using it for years and it’s excellent.

Anyone know if this an improvement over the JuiceFS solution, and in what way(s)?

Same. That snap-by-default issue (last year?) pushed me from kubuntu to pure Debian and I haven’t looked back. Technically you can just remove snaps from Ubuntu to get around it, but it’s an incorrect assumption that users want that forced down their throats when upgrading in place. Left a bad taste - Debian is my goto stable OS now.