How about Flare allowing at least 2 simultaneous upgrades on waves? Under normal circumstances it takes ages (~1.5 years might be more with the coming of wave 9), allowing wave upgrade (max.2 simultaneously) will help to reduce that gap.
Is it not possible for Flare to allow building and items inside it to be upgraded at the same time? Eg. If you upgrade troop academy, you have to wait for ~6 days before you can upgrade the troop. I am suggesting both working together. Keeping in mind future updates, where new levels and troop, spells etc. can be added, this will be a healthy addition.