I agree with so much of this!
Adding a sun cost to merges is a big one for me. Otherwise, you'll just have players with 1000 sun and nothing to do with it. Tier 3s should cost that extra sun.
I can see the game surviving with levelling in it, but I think it does need major reworks to make it actually fun. Right now, each Plant level scales at WAY too high an amount.
As for sun producers, I completely agree that Sun values should stay at fixed values, 15, 25, 50 etc. for one big reason, and it isn't nostalgia. When I pick up sun, I should actually be able to predict whether I'll be able to plant based on the sun I pick up. If that number is 32 or something that changes every time I level it up, that simply removes that aspect of the game. Right now, Sun gets picked up blindly and the numbers don't matter anymore. Levelling up sun producers could instead quicken the sun production speed, reduce the time that the FIRST sun appears after you plant it, increase the HP of the plant, or produce an extra sun after X productions, depending on the plant.