Howdy, So the initial version of this mod did exactly this,
there was diminishing returns on the seeds, however there was a another recipe utilising uranium to up-cycle the seeds, so you could use a regular farm to get seeds, up-cycle them reliably to legendary - then farm with them. Alternatively, you could also use maxed legendary prod modules to break even with seed production. And that underwent several balance iterations. (This is all pre version 1.0.0)
That first version was specifically coded for Jellynut and Yumako seeds. However I really wanted to expand my mod to include my other mods - or anyone elses mods. One issue with this was the procedurally generated content, and how to balance seed making (as there's no easy way to garuntee that the seed production mechanism is the same for modded plants etc etc, so I just used the base tree harvest recipe, and called it a day.
For this reason, you get the unbalance you mention. I get it, I've been playing with the new balance and it is rather easy to get the legendary stuff once you eventually set it up correctly. And I imagine even easier once you setup the space cultivation! But i suppose the goal of the mod switched from balance to more fun in general - or perhaps to just making a Gleba / Gleba main run a primary part of the game type.
There are a few ways to address these issues:
Alter the existing recipes of the mod for yumako and jellynut only - as they are the ones that are properly predicted - to be balanced such that you don't get enough yumako fruits per craft to get a return on seeds without more productivity.
(This would likely require altering the pentapod seed iteration recipe, as it would take carefuly calibration to ensure you get enough of seeds of each quality to craft enough nutrients to continue seed upcycling.)
Make it into a separate mod with the balance you describe, which is simple about balance
Make some mod settings to control the total amount of Cultivation outputs per seed - so it can be customised to your desired difficulty.
I have been thinking about it alot, and I think addressing mod balance with settings is a good way forward, as there are many people who enjoy it a bit easier to farm their legendary factory, but your quite right that in its current state It is a tad easy and over-shadows the other planets (Although to be fair in the games vanilla state, Vulcanus is too easy and overshadows the other planets anyway haha)
What do you think about some mod settings to control the total amount of products from a cultivation.
What do you think about more researches to increase the Cultivation productivity?