This might be a bit of a controversial opinion but honestly I think a better reward would be a touched up version of the final demo Castle Eggman Zone with a new name, slopes, better textures, that kind of thing. The "button hunt" formula for level progression might not have much of a place in the main campaign, but I think those two levels as a bonus unlockable zone would be pretty cool as a reward. I don't actually expect it to happen though, especially since we already have a CEZ themed level to unlock.
In regards to Techno Legacy specifically, I don't hate it but I would prefer that if they are going to go for a nostalgia unlockable like that, either overhaul it to feel more like it belongs with the rest of the game or keep it just like how it was so that it's properly nostalgic and doesn't feel "wrong". The problem here with Techno Legacy is that it's is changed just enough to feel more like a crappy edit submitted by some kid as their "first map" after altering the old THZ2 for 5 minutes rather than a proper historical homage to the act.
I can understand the thought process behind making the chemicals a different color at least, so that players who didn't play the original stage don't accidentally jump in and hurt themselves thinking it's like the newer goo that makes you bounce. But at least give back the original enemy placements, that one emblem that requires going into the chemicals to get into the pipe, and change the barrels to look like they have green chemicals in them instead of purple.
Alternatively, if you are willing to put in the extra effort, give the act the proper touch up treatment. Add some slopes, implement some better textures, maybe change the shape of the rooms and hallways a bit, spice up the enemy placements and add some of the newer enemies in where they feel appropriate, place some pipes along the walls, etc. Overall make it feel like the old THZ2 layout made under the newer design standards. That would make Legacy feel more like a proper reward.