As a developer (not into bots), I can say that for sure it is possible to create a bot for even more than 10.000 accounts like you said 100ks. And they exist, but people created it for themselves and hardly gonna sell it.
So either you find those people and beg them to sell it (really hard to happen) and will cost, or you find an experienced developer to make you one but again this would be even more expensive & hard to find.
And even if you have a good bot it doesn't solve the problem with the good quality account, proxies & Instagram algorithm, especially on that account numbers...
I would rather go with 100 accounts at the time, find a good source of quality accounts & proxies, and then scale it a bit, for example, you can run jarvee with 100 accounts on multiple computers, etc. Something is better than nothing right?
But like someone else said it's really hard to work even with 100 accounts, maybe this ^ not on Instagram but some other social media that are less popular?