Hi sven! Loving the diplomatic improvements in Steam's dev branch.
I add all the slots I can then terraform the planet for more pop. That also usually adds more slots.
The tedious amount of clicking is caused by the queue fillers. The +1 slot techs are fun and simple as they are. Your proposed change doesn't sound intuitive.
Noticed I have to add/remove queue fillers while waiting for pop to grow. End up periodically checking many of my planets to see if a slot has appeared.
Finally getting access to a few million of a new race is just about as good as a +1 slot tech. It just happens over more turns. Not having aquatic or subterranean races available is definitely a hindrance.