Here's a simple? Script that adds in Berry Trees like SwSh or Sm to your game.
to call it use berrytree(berry1,berry2,berry3) where berry is the name of the item you want the
tree to drop, you also need to add an encounter type "berry tree" for the pokemon that will appear if you shake the tree too much
Here's the script : https://pastebin.com/index/dJxy1T2H
I would suggest using it along timed event for the true berry tree fell
I also attached a berry tree sprite I did if you want to use it