Vecna's early victims saw a grandfather clock. Which in symbolism may have been similar to a defcon level, doomsday clock. The first unlock level was 4 victims. The second was 12 which may have been foreshadowed by the clock having 12 digits.
I too thought they did a good job with the finale. The main disappointment for me was the kids peaking at 17 years of age and not being able to carry their friendship, bravery and achievement into real world success.