While I like the threaded-rod (slow to reset though) and LED bar graph approaches to displaying progress, the water column is not difficult or non-linear. You pour water in the top of the column through an adjustable-flow irrigation dripper (50c), quite-constant flow rate. If you don't want to see the water flowing down into the column, you make a U-shape column with open tops and one arm hidden, pour the water into the hidden arm. You can put your float sensor in the hidden arm too to detect winning.
On further thought, latching solenoids are a bad idea because they must be manually turned off; you don't want your system to power-up in an unknown state and you want everything to just stop if there is an issue with the software. All of those things can be overcome but it's unnecessary design pain just to save 20W in a system that contains a probably-1000W pump.
PS impact sprinklers make awesome machine guns.
While I think running this from a reservoir is great, the quantity of spillage means you probably still need a mains water connection with a float valve to top-up the reservoir.