Efficient progress depends on three parallel queues: builders, the Laboratory and heroes. Before spending, check which one is likely to become idle and choose an upgrade that keeps all three moving.
Prioritize offense
Army Camps, the Laboratory, Clan Castle and the units you use most improve your ability to earn resources for later upgrades. Hero upgrades require extra planning around important wars.
Prepare for the next Town Hall
You do not always need to finish every secondary upgrade first, but avoid moving up while your main army and essential offensive buildings are far behind. A clear rule based on actual levels is more useful than an invented completion percentage.