Over the timespan comparable to a mortgage? Broad-based equity funds.

The worst ever 20-year return for S&P 500 was +6.4% per year for the 240 month period ending in May 1979.

https://www.thebalance.com/rolling-index-returns-4061795

*assuming conditions over the next 150 years mirror those during the sudden rise of a single global superpower with control over the world's reserve currency and a continent full of unexploited natural resources at its disposal

For that contingency, you are going to want to be forming lots of political allies with lots of guns.

Not exactly what you're looking for, but in the US I series bonds are currently paying >7%. The biggest downside is you have to hold them for a year. The other issue is you're limited to buying $10k in a calendar year.

Currently paying. It is a variable interest rate, and it would behoove you to assume that if a treasury is paying 7%, then real inflation is much more. Hence the SP500 returning 30%.

Sure, but if you're looking for the lowest risk thing that's paying at least CPI inflation. A lot better than cash under the mattress (or a savings account for that matter...)

I paid mine off years ago. I got the same thing then. "Name me one investment I can put this 100k into that will pay 1500 a month". That is closer to 15% per year...

Your actual return is only the interest portion of the payment, so that's an odd way to calculate it

I get what you are saying. It is fair. But my goal was to have no 'payment'. So it would need to generate enough to cover the whole thing. It was like getting a 1500 per month raise.