So I gave planetary interaction another try. Have a setup now that looks like it's capable of generating ~7 million ISK/month. Yes, per month. Which is not very exciting. Guess I'll go through the guides again at some point, experiment some more and see if I can make it worth doing. Apparently import/export costs are high enough that in some cases certain products can be net loss. Costs of exporting a material from one planet to import to another to be used then the result exported again really add up, and the extra time overhead of having to move things around doesn't make it better.
I see how this could possibly be lucrative. Add a few skills levels so I'm building at more planets, upgrade command centers more, stick to products that can be manufactured using only a single planet's resources to reduce export/import costs, pay a little more attention to what's selling...but overall the entry bar for PI seems kind of high for a result that's not all that spectacular. Yes, there's a certain appeal to having "of passive income" but it's not really passive. At present to do the 7 million/month, that's assuming a 12 hour cycle. Yes, presumably that could be improved...maybe it could become a couple times a week thing, but if I'm spending an hour, 2-3 times a week to make even optimistically 50 million ISK/month...I'm not sure that's worth the time investment. It would be just as effective and a lot easier to mine or do missions with that time instead.
Maybe it gets better with more skills, more planets, and a couple dozen more millions invested in command center upgrades, but overall I'm not really seeing it.