The problem with your step 4 is a limited data set.

NES has been out for almost 30 years and shows no sign of slowing down with pricing, and Master System is actually continuing to go UP in prices within the last several years actually.

I'm not saying all games will continue to rise no matter what, but the difference between Atari/Coleco/Intellevision and NES/SMS is quite large in terms of game quality, quantity, and variety. In my opinion, later generations will hold their own better than the very early ones because of their continued play-ability, the only exception to that being early/bad 3D games.

As for when to buy games, get them when they're cheap enough for you to not balk at it. There's no magic formula involved in it. If you're going to try and time everything perfectly to get the maximum return, go to the stock market instead.