Assumptions:

Body doesn’t age, you don’t get sick nor carry disease, while you feel pain you can’t be injured, you cannot die, and you won’t succumb to any mental conditions such as extreme isolation for millennia.

I would work a high paying job to pay off the debt (say Saudi ARAMCO or Oil-rig), then get a degree in a high paying field, and save at least half my income into global stock indices that average maybe 7-10% a year. You would need to move every 20-30 years to avoid “why isn’t he getting older” but after 100 years you’ll have enough money saved up you can just live on your investments.

Then I would use my wealth to start nudging humanity into space travel so I won’t get stuck on Earth when the sub expands… also so humanity doesn’t die out when that happens. Otherwise use my wealth to help stamp out poverty and illness rich-dude style.

I have three big fears:

One is humanity would get wiped out before space travel to other stars is possible. If this happens after AI come on line this might be a workable problem, but if it happened before then there’s a real chance I would be stuck on Earth for millions of years trying to develop the AI to assist me in developing a spaceship that could get me to another planet. Even if this is possible I’ll be all alone, but who knows maybe in 50 million years I’ll be able to create my own AI that can keep me company, or I create something that creates something a little better, that creates something better, and eventually we have AI… just have to hope it doesn’t want to throw me into space.

Two is eventually being ‘found out’ by the CIA or whatever group and spending essentially forever being tortured. Honestly it sounds easy, you just keep moving, but over time facial recognition makes hiding harder, and since surgery doesn’t work you’ll start spending more and more time “hiding”.

Three is everyone I love dying. Even if I get over it, fall in love again, and they die. Again. Maybe someday AI becomes so advanced you could fall for an AI and not worry about losing the love of your life for the 500th time.