The otherside of this economy is the ROI of the account. If you paid money for the account, or for promos, or analytics tools, you should be able to know how fast you can break even doing your own promos. Can you account charge $10 or what.
When YOU want to do promos for other people, you should already be in a position to DM meme accounts and individuals directly.
The individual market has its own tiers, anyone under 10k followers is out of the loop, but is doing okay at the social popularity game which is the crux of instagram: A follower casino where few people actually win. Many will pay $10 for a promo. More will do a reciprocal promo for free.
Regarding private profiles, people want to know what they are missing. Is it explicit and erotic tantalizing content that will be short lived before it gets taken down!? Is it hilarious content that only the cool people get to know about!? Maybe there is a sneaker or consumer electronics promo happening on this secret page, maybe a secret society that conveys long lasting advantages if you happen to be there. Yes its an instagram culture thing.
Yeah what to actually post on the account is hard work. But you don't actually have to do it much if you just want a profile with authority and power in direct messaging or real world trading for free stuff (you promise exposure, real luxury businesses give you their service). But for active posting, there are various tools to automate it, but it is really hard work either way. If you want to create a very valuable account to generate cash flow from more expensive one-off promotions, to funnel many users to a real subscriber business of your own or something you partner with, or selling the account at a revenue multiple, then you have to figure this out. You have your niches and you can have original content or not. Being able to quantify 3% engagement is a gold mine. Higher? Palladium.