[Resource network connections are] nicely illustrated by the colored star lanes. Some sort of range-limited growth effect wouldn't be (weather calculated by direct distance or starlane jumps), and would be less obvious, more confusing, and/or probably require a toggled overlay to try to figure out what is going on.
It's quite tempting to make other range-limited effects that wouldn't be appropriate to extend over the whole resource network... So there may be need for such UI tools regardless.
Perhaps food as a resource could still be used mainly to boost target population, but with a local spending limit for each planet. A spending limit for PP on a planet would also be useful to have for balance reasons, and I think has been discussed.
Import capacity for each movable resource might be determined by something straightforward... Perhaps industry produced on a planet determines its max PP spending at that location, but minerals may be produced locally or imported? Similarly, food would have some limit on how much can be applied to a given planet to boost its target population - perhaps the health meter? This could probably be done without a separate target and current health meter, since the effect on population is to alter its growth / decay rate and stable state by moving the goal, rather than causing an instant change, so there would be no need for a similar smoothing of the changes in health itself.
Still seems kludgy / complicated, but I'm not sure how else to determine how much food a planet can import. It needs to be limited in a very location-dependent way, since you don't want natural size 1 planets or size 25 planets being able to boost their target populations by the same amount all the time.
How this will work for an isolated planet is also interesting... Presumably planets will produce food at a rate dependent on their population in many cases, so there would be a feedback between food output and target population, and thus population growth rate, for planets that are growing while focused on "food" or "growth".
It does need to be called something other than "food" though, as it wouldn't make sense to have planets naturally have stable populations even when no food is available, and then have an increase in stable population when food becomes available...
Much of this above discussion makes me still want to have limited-range non-stacking boosts to target population when connected to a growth-focused planet. This is a mechanically quite simple system (few meters, no self-feedback issues) even if it requires some additional UI to illustrate the ranges where bonuses are being applied...