For the fune tuning, some changes involve upgrading the bpool to version 5000 but disabling some features. You need also to do that for rpool and align with our defaults. You can see them for both rpool/bpool on zsys-setup « scripts - ubuntu/+source/ubiquity - [no description]. Warning: -O sync=disabled shouldn’t be put on finale dataset, this is only to speed up installation (we reset it to standard after the installation: zsys-setup « scripts - ubuntu/+source/ubiquity - [no description])
There are other changes, like we always have an ESP partition now since 20.04, even if you booted in legacy mode. We install grub there instead of a dedicated partition and bindmount /boot/grub from /boot/efi/grub (more details in the script I pasted above).
I think that should do most it, but it means playing with partitions and such. Ensure you boot once as zsys will migrate some tags from the older scheme to the newer ones 