Back in days gone by, I was curious as to how Tech 2 ships and modules were produced, so I started reading around. Turned out to be a relatively arcane method, but fundamentally you needed to invent a Tech 2 blueprint from a regular blueprint, using invention, an interface and datacores. Now, datacores turned out to be a commodity that at the time could only really be obtained through research agents. Agents have changed a fair amount since then, but research agents have not been removed, even if they no longer generate datacores quite as quickly or cheaply.
Research agents basically churn out datacores, worth around 100,000 ISK a pop depending on the type, with no input from you. So they are as passive an income source as they come, apart from taking the cores to a market hub. You could of course try invention with them too. The down side? You’ve got to grind standing hard to get access to the higher level agents, and there aren’t loads of them. But, doing PvE for the relevant corporation won’t be too much of a hardship and will after all get you loyalty points.
One note of caution is that even where you have really good standing with the relevant corporation, bear in mind that if you do what I recently did (!) and hurt your faction standing you might find yourself unable to access your research agents. Diplomacy is your friend in this scenario, and you can always access Level 1 agents of the faction to drag your standing with them back up again. Just something to keep an eye on.
One last aside, invention can be a passive income generator like manufacture, but you need more skills behind you and sometimes invention produces nothing at all at a cost of the datacores invested. I have gone so far as to manufacture my own Tech 2 ships in the past, from blueprints I invented, but it requires a fair bit of running around to get the ingredients and was purely for the “I’ve done that” of it!