So the big complaint with the garden seems to be that some plants have very erratic growth, and especially plants like Juicy Queenbeet and Drowsyfern.
A minor thing I've noticed is that Seedless to nay doesn't quite work as advertised, since not all mature ages are divisible by 20. For instance, Baker's Wheat matures at 35, which after Seedless's reduction is 33.25, but that's effectively 34--a much smaller reduction than 5%.
I would therefore suggest a Heavenly upgrade that is only available if you have all five Garden achievements (harvest 100/1000 plants, plant 36 plants, complete the seed log, sacrifice the seed log) and a complete seed log (giving you an incentive to complete it again once you've sacrificed it). This upgrade would load an alternate version of the Garden minigame, in which growth is more consistent.
Specifically, all age variables would be multiplied by 20, except for maturation age, which would be multiplied by 19 (per Seedless to nay's 5% reduction). This could be any multiple of four, since all mature ages are divisible by 5, but 20 makes Drowsyfern's base ageTick variable 1, so it will always grow on every tick. Plants would age from 0 to 2000. I also want the multiplier to be divisible by 10 so that plants like Elderwort and Cronerice have integer ageTick and ageTickR variables, so it must be divisible by lcm(4,10) = 20.
Note that Juicy queenbeet would still not be guaranteed to grow on every tick; there would be a 1/80 chance of its not growing. Also, when the slow-growing plants grow, they would not jump by as much as they currently do when they grow, but they would grow on every (or almost every) tick. This is another reason to put the multiplier at 20; any larger and Juicy queenbeets would grow on every tick, and given their reward, I think it is suitable to have them remain the only plant not absolutely guaranteed to grow.
This would not make faster-growing plants grow more erratically, since the minimum and maximum possible growth would, relative to the total age of 2000, still be the same as it currently is, but the broader range of possible growth would reduce the variance in aging relative to the total age.
Another possibility, either as a separate upgrade or as part of this one, would be an upgrade that increases the chance of mutations. That might only be unlocked, perhaps, once you've sacrificed the seed log a certain number of times, similar to how Sacrificial rolling pins is only unlocked once the Elder Pledge has been bought a certain number of times.