So a note on why the agriculture guys are going apeshit over the traditional agrarian society expansion route. I will remind that this is a fantasy setting. The progressives as you call them were taking the traditional route that agricultural societies do, which broke with the sort of fantastical 'tech' that the traditional guys were using.
That's not to say it didn't have a lot of advantages over how the traditional guys did things... but it has some pretty obvious downsides too
I'd take note that the Weilu are in the completely wrong biome for traditional water and silt intensive agrarian development based on grains cultivated along floodplains. Its heavily forested, and prior to the Bais developing Bronze, there was no practical way to clear land for that. And that early grains were pretty crappy to subsist on, they were pretty tiny and hard to digest.
What they DID have was:
-An abundance of fruit and nut bearing trees, bushes and vines.
-An abundance of foraging animals to convert inedibles into meat.
-Very little 'free' water thats not taken up by the forest, but no arid areas for the same reason, the forest will conserve water within biomass.
So a possible approach for them to start agriculture was:
-Discover fruit or nut bearing trees.
--While previously they'd have harvested and moved on, the cheat was simply to make a deal with the plant's spirit, to help plant more of it and to swear off harming the plant, in exchange for getting larger bounties on a regular, seasonal schedule, which was probably innovated with a Calendar such that theres always some plant providing sufficient food by Tsu.
--Once the system was set up, the 'human allied' tree crops are going to outcompete their rivals unless they made similar deals, functionally dragging the spiritual ecosystem into one that favored those who pacted with the mortals. Think WWI-esque chain of alliances.
--With that, drag the spirits of weather and rivers, etc into the same chain of pacts, in exchange for services rendered, they would provide sun, rain and flood at the right times every year, rather than whenever they feel like it.
Basically, leverage humans as the middleman broker between spirits, using the services of some spirits to pay off others.
This would work fine with a migatory cycle to have the tribes live wherever there was more food that season.
But if you yanked out a plug on some points, like clear cutting trees for land to cultivate more productive crops, you're going to break the pact cycle where by the river floods on the regular as long as you tribute it its favorite fruit every year. A fruit which isn't available anymore because you cleared that grove
...so you dam the river and get it to irrigate the crops on YOUR terms rather than be held hostage by some sulky spirit. Which probably didn't play well with everything else up and down the river as the river's spirits are displeased, but which DID mean that instead of one harvest a year you could manage three.
As to how the ancient Weilu could have cultivated, if they didn't use spirit stones and for the sake of this discussion we assume they don't. My current theory is that Tsu would have needed resources to cultivate himself to the point he could work out the seasons and tea and be the Diviner and introduce agriculture as a really cool and good thing. So my guess is there are medicinal plants that provide the same function as a spirit stone as an energy source by collection of easily accessible qi in their tissues and seeds or the like.
Alternatively spirit pacts and spirit cores. They're spiritblooded, they can eat cores raw, and the Sun/Moon are patrons sufficient to get to pretty high level cultivation if you can meet their standards.