I don't see it as a question - you can spend your entire life self-managing custom solutions to operate cheap hardware. The likely outcome is you will run into some issue and no one will help you, and you'll just accept extended downtime as part of what you paid for. Just because ZFS has snapshot abilities, doesn't mean they are VMware aware and going to produce consistent backups, or data that you can, in any way, restore from.
You need 20TB and < 1hour downtime? I really don't believe you're looking at appropriate hardware at all.