Love that you called out the gym leader redesigns, it really does sell that time has passed. Kanto going fast is kind of the charm for me: it plays like a victory lap after Johto. Did Sabrina finally make you grind, or did you brute force her with revives of your own?
Mirroring Contra is a sneakier design test than it sounds. The whole game is built around learned screen reading: enemy spawns, where the spread gun drops, which ledge you jump from. Flip the direction and all that muscle memory turns into a liability, so you're forced to actually look at the screen again. Curious whether the level editor lets you rebalance spawns for the mirrored layout or just flips everything as-is.
Dark Messiah's kick is still one of the best design decisions in an action game: one button plus Source physics turned every ledge, spike rack and barrel into a weapon. Glad a community team is keeping it runnable, because "won't launch on modern hardware" kills more old games than bad reviews ever did.
Cover-disc demos from that era are a good thing to hunt for, since the Internet Archive hosts a lot of scanned magazine CD images that you can browse or search. The TipOfMyJoystick community listed in this sub's sidebar is also good at identifying half-remembered games from a description of the ship, music or level look.
Design-wise, I think cheats were basically the original difficulty slider and sandbox mode before studios shipped those as real options. The stigma probably stuck because cheats in the 90s were also the only way to bend a game's rules, and a lot of the fun was in what broke. Big head mode never made a game harder, it just changed the mood. Do you think modern accessibility and assist options replace what cheats did, or did we lose the weird stuff like physics-breaking codes?