What interest? It used to be that you paid for someone to hold on to your money.
Yes, passive compound interest has sadly only been around for about a century or so and can’t be guaranteed to last a millennium in any given version of society.
Basically, if you can expect to live several thousand years or hand down plans over that sort of time frame, it’s not much use for planning unless you also have some way of reliably predicting the future. For instance, if a modern person is thrown five thousand years into our past and gifted with functional immortality, then depending on their talents and knowledge of history they may be able to ride or manipulate historical events to their benefit. But they can also expect to endure millennia of starvation, disease, and other deprivations while waiting for the next cool historical event to come along, assuming they haven’t changed history by just being them.
As for actual ideas - I’m reminded of one of my favourite sci-fi authors, Kage Baker, who sadly died 15 years ago. Her main work was the
Company novels, which start with the premise that in the next few centuries, somebody (Zeus Inc, AKA the Company) work out two technologies - functional immortality and time travel. However, the former involves some very painful cyborg surgery and gene therapies and can only be done in children; the latter is extremely limited and cannot change history. The Company therefore monetises these discoveries by travelling back in time, choosing children who would have died in various disasters, and turning them into immortal indoctrinated employees whose main purpose is to preserve lost artworks until the present day for sale to collectors.
(The plot, of course, comes from the fact that this is a clever use of two limited technologies and can be very lucrative but is an incredibly terrible idea in that it creates an army of disaffected immortal cyborgs who think that after several millennia of working for the Company and enduring any number of traumas, they should get some slice of the pie when they reach the 24th century.)