I’m sure there’s plenty of activities that you throw large quantities of isk at to get even more isk out. If you want to manage like 300 characters to do PI + SP farming, you’ll get more and more money for each character you buy. It requires a huge startup capital, but once you get going you can generate enough money to buy more characters to keep making even more money.
Industry and reactions are also pretty profitable like this, the more characters you train up the more slots you have to do industry to keep getting more profit to buy characters with.
The only problem is stuff like this burn people out very quickly, imagine spending 5-6 hours a day having to log in, collect stuff from reactions, sending it to a main market character, starting up another reaction and repeating the cycle for 300 characters
I personally did this with SP farming and PI grinding for about 3 months on 3 characters, and even though i got about 1b/month profit + 500 plex every month, I quit after only 3 months because setting up and monitoring daily market sell/buy orders for PI + moving it around is extremely mundane.
Most people just opt for buying more rorqual accounts and mining though, which also makes more profit the more money you throw at it