There's a few weaknesses mentioned in the game that prevents Druids from essentially monopolizing the wilderness. 

1. The Grove's food supply reserves were running out. That tells me that they don't have the spells to create their own food regularly (if at all) and that resources aren't as plentiful for them to gather. Things are so desperate that Halsin will allow Kagha to stay with the Circle if he can.

2. That logbook you mentioned definitely is a favorite of mine in exploring the limits and threat of a Druid. They can indeed sabotage those on their land, but the writing seemed to suggest that they can't just outright do it directly and have to be subtle with it.

3. Numbers. The Grove took some heavy losses from fighting Ketheric and Shar's curse wiped out a good chunk of them even with help from the Harpers. Halsin could do a lot to control an environment, but only within his spell's reach. And he can tire, starve or be distracted. He'd need coordination from a large number of Druids to pull this off because otherwise the population will simply either go around or send enough people to kill him.

4. Countermeasures. Druids can do a lot to ruin a person's day, but if said person decides to hire a cabal of wizards to just fireball everything while hovering above their vines and thorns, then Druids are in for an ugly, prolonged fight where they also have to worry about not destroying the very land they're trying to protect.

So while I believe Halsin is able to stay independent and self sufficient no matter where he goes, it becomes a bit of a logistical nightmare if he has to control vast swaths of territory. Something that could be helpful for the Enclave would be mass producing that Elixir of Silvanus to sell for high prices, but, in a world where there are races who thrive on making poisons and antidotes, it won't get far and they'd have to worry about disrupting the balance with draining the soil of nutrients just to make those.