Indeed, it’s limited in the first installer implementation. However, nothing prevents you from doing your custom installation via debootstrap or any other form. The system itself handles whatever layout you wish. The guided/help installation (native to ubiquity) will be as described above though in its first round.
Those aren’t related.
As told, via a manual installation like debootstrap, you can install 19.10 on existing zfs datapool. However, I doubt your layout will be compatible with zsys and you lose all advanced features (basically, you have zfs as of today, and you need to handle it manually).
On the other questions, the advanced grub support we introduced in 19.10 have multiple system support, as you have already seen and discussed above.
The installer option should be released once reviewed. We got final design last week (due to an issue in the ubiquity codebase preventing implementing previous design).
Zsys will stay in universe, meaning: not installed by default, but an option for those who wants to experiment with it.
There are 2 contestants in the room here:
- lvm + luks + zfs. The underlying (lvm + luks) is already natively supported in ubuntu for TPM, so the goal would be to reuse the existing code. However, we plan for next week with the kernel team to have performance comparison so that we know where we stand at.
- native zfs encryption, which means reimplementing the whole integration with various components (plymouth, grub, the initramfs…), which is more work. Also the encryption is fresh from 0.8 and we need to evaluate how stable it is.
Anyway, those items will be for the second phase, once zsys is a little bit more mature.