Hate? Are you kidding me? That’s awesome! And Gentoo is not going anywhere, it’s still waiting for you :)

So in a bit more detail:

• Create a btrfs partition - I sometimes use the whole drive + usb for boot, sometimes carve out a boot partition. Up to you.
• Create desired subvolumes
• Install Ubuntu, since that sounds like the one covering most of your requirements, but try avoiding fully automated route - specify the subvolumes so you don’t have to migrate later on. Or at least don’t have to migrate everything.

I’m going to assume the whole drive is encrypted using luks and there’s a btrfs partition on top of it. My btrfs layout would be something like this:

root of btrfs (subvol id 5)
|___Ubuntu (directory)
|   |_@root (/, not /root)
|   |__@home
|   |__@srv
|   |__@var
|   |__@opt
|___Debian (directory)
|   |__@root (/, not /root)
|   |__@home
|   |__@srv
|   |__@var
|   |__@opt
|___Qubes(directory)
    |__@root (/, not /root)
    |__@home
    |__@srv
    |__@var
    |__@opt

I’ve never played with Qubes, so I’m assuming the layout here.

Your fstab would have to specify the subvolumes in the options bit of the line:

UUID=something-something      /               btrfs        defaults,relatime,subvol=Ubuntu/@root    0 0
UUID=something-something      /home           btrfs        subvol=Ubuntu/@home    0 0
UUID=something-something      /srv            btrfs        subvol=Ubuntu/@srv    0 0
UUID=something-something      /var            btrfs        subvol=Ubuntu/@var    0 0
UUID=something-something      /opt            btrfs        subvol=Ubuntu/@opt    0 0

Yes, you specify the same device (luks one, in this case, not actual /dev/nvme0n1p1) multiple times. BTRFS can only take mount options once for the whole filesystem, so specify whatever you need for / as anything else other than subvol will be ignored for the following mounts of the same device.

I would go with systemd-boot for the bootloader or, better yet, unified kernel image, but that’s beyond this post. In general, though, you should either pick one OS to control the bootloader, or have separate boot devices/partitions for every OS so it can do whatever it pleases.

Once you have one system running - mount stuff somewhere (/mnt comes to mind) and install the target. Gentoo has good instructions to make sure your chroot env is not missing anything. I’ve used debootstrap before for Debian - it worked well.