Buy e.g. a three-month Treasury bill directly. These are fundamentally zero risk, being backed by the full faith and credit of the US government. The catch is that yield is paid only when a bill matures, and to achieve the yield we've listed, you'd have to invest in a three-month T-bill. The high yield is guaranteed, but only if you're able to wait three months at a time to access your cash.
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Large corporations using treasuries for their cash reserves will have dedicated staff managing multiple, constantly-maturing T-bill investments to guarantee some liquidity, as well as facilitating trades if cash is needed sooner. Trying to replicate this at a startup means spending far more in valuable founder time (or actual costs on expertise and infrastructure) than what you'll earn in interest

“T-bill funds”—managed mutual funds composed primarily of T-bills—that do for clients what large corporations’ in-house teams do for them, though your return will be diminished somewhat by the fees paid to the underlying funds. Your investment will still be susceptible to price risk, too: the mutual fund is constantly reinvesting as underlying bills mature, and if you want a large amount of cash back immediately, you'll be forced to sell bills that haven't fully matured yet at a discount. Cherry picking just the mature ones isn’t an option