Questions about true ending

I have a few questions about the true ending.

So I'm pretty sure I've given Shennong all the poisons – he has gone to the Galactic Dock and is now the leader of the apemen.

Main thing I want to know is, does the game give you any concrete indication that you can now progress and you'll get the true ending? I've done the normal ending already and I don't want to get locked into fighting Eigong again unless I'm sure it's for the true ending.

I played a game of that chess-like game with Shuanshuan and Yi said it was likely the last time they would play together. Could that be the indication?

Also, I've grown the tree to what I believe is its full height (I gave Shuanshuan two GM fertilizers). Do I have to do anything else with Shuanshuan? I'm not 100% sure I've given him every artefact, as I don't know if there's any way of checking you've done that.

I've seen some people mentioning getting an achievement. I'm playing the Switch version, and I'm not sure if achievements are just on Steam (I didn't get one when Shennong went to the Galactic Dock).