Not investing such a large percentage of their capital in long-duration fixed income vehicles all at once.

There's nothing wrong with going long on duration, it's a hedge against decreasing rates. The problem is when you go all-in on long duration investments and rates suddenly shoot up like they did, you now can't sell those assets without eating a massive loss.

An appropriate hedge would have been doing what every retail bond trader does, build a ladder. If they had simply bought a wider variety of say 1/2/5/10 year securities then they could have let the longer-dated ones sit and sell the shorter duration ones (and they wouldn't have suffered such a huge loss of market value that spooked depositors and started the run in the first place).

They took deposits from depositors who would blow up if interest rates went up, and then used those deposits to buy assets that would blow up if interest rates went up. Interest rates went up, so their assets crashed at the same time that deposits plummeted and withdrawals skyrocketed.

If you want to standardly hedge against interest rate risk, that's what swaps are for. If you want to take on a comparatively less rate-sensitive portfolio, then you buy shorter-dated bonds. They yield less, but surely that's better than "the FDIC seizes your bank and your equity goes to zero."

Interest rate swaps is one solution.

Here is a primer: https://www.pimco.com/gbl/en/resources/education/understandi...

This is another (PDF): https://www.treasurer.ca.gov/cdiac/publications/math.pdf

Buy T-Bills or other short maturity assets instead of long maturity T-Bonds and mortgage-backed assets.