To be a heretic as I always am........
How can you have to much cash? Honestly! There are times when I'd love to have more cash!
But to be helpful..........
Rush-Buy a bunch of Warships and have them patrol your slice of space for whenever it would be necessary.
Like to launch an invasion on someone. That spends the cash.
Bribe others into wars. And if you're really bored, bribe the others to make peace.
It's fun to run galactic politics from backstage.