ubuntu 20.04 lvm snapshots during install?

I´d like to switch to ubuntu 20.04 after release and was leveraging a preview today.
During install I tried to create a custom storage layout with LVM but never got a "save to disk" / "continue" like button - is this to be continued?
I then switched to "use an entire disk", marked "set up this disk as an LVM group", but was unable to size my root partition - it was defaulted to 4GB (ca 10% of the 40GB virtual disk I allocated). Maybe the 4GB is just the minimum for installation and I know one can change it afterwards, but I increased partitions twice in the past and it was always a pain to me...

Let me take a step back and explain my goal. Consistent backups of systems are easy with windows, thanks to volume shadow services being standard. I´d like to go for something similar with Ubuntu as well, and the closest I am aware of is LVM snapshots, or probaly btrfs snapshots. The hard part is however getting the file system partitioning right from the very beginning. If there were an easy option to reserve 10 to 20 % of the capacity for snapshots right from the beginning, this would ease system administrators tasks to set up backups significantly. And in fact it looks like not a lot is missing, maybe some testing and fixing..

Obviously there could also be a tool/script/good documentation how to do this after installation.

Thanks for your consideration.
Best Regards,
Joachim