That's not the biggest borrow in history by the way. Not even close.
Whats important when investing is knowing what your risk-free rate of return is. And in cn the risk-free rate of return is HIGH! For every dollar invested in a smaller nation (under 4000k infra) investments usually amortizes within 30-60 days.
That means you'd have to offer a RISKFREE rate of return of about 640% per year (100% in 60 days) . As your nation is not in an alliance, does not have any collateral (infra in this case, or reputation) I would set the probability of you being able to repay this loan to about 10%.
That means you'd have to offer someone 6400% percent per year to make this thing interesting.
So RIGHT after the last aid has been payed to you, you'd already be in debt by ~ 110mio. (12* (sqrt36(64)^6-1)/(sqrt36(64)-1)*sqrt36(64).
By the time you make your first payment you'd be in debt by ~ 123 mio. (110*sqrt36(64)).
If you wanted to repay the debt only using 4 slots and no tech, it would take you 120 days at a total of 144 mio.
(this of course being the best possible case for you, as 10% chance of payback, 640% rate of return are estimated in your favor and the other nations are putting in a lot of work without even being compensated for it.)
