But that doesn’t avoid the restart problem… If you have a restart while any of the switches are on, or one of the switches goes dark (power or wifi out or …), this will stop the automation as the switch before isn’t on for 15 minutes… What could help would be to check for “from: off”, but not “to: on”.
Anyway, I wouldn’t want to work with a fixed start time…
BUT, what I haven’t thought about before, have you taken a look at the various irrigation integrations? One of them can even calculate the water needed to get an optimal result and safes water on the way… 