As a Googler, I have this pain point. I was envisioning a product that sweeps the leftover income at the end of the month after all credit card bills and rent are paid, into a brokerage account. It would also automatically invest that amount into my choice of ETFs, which Vanguard bizarrely doesn't seem to support today. This seems to achieve the same goal with a simpler implementation, and then as you improve your cash flow predictions, you could start keeping a cash buffer and delaying investments to avoid some cash<>asset conversions.

Worrying about capital gains taxes is a head-fake - you're only paying tax on your gains, which would have otherwise been minimal interest, which btw is taxed higher than gains. The risk here is just that the amount of margin interest I pay is dependent on how quickly you're able to sell, although hopefully this shouldn't be a massive issue for VTI et al.

Your messaging on the website seems clear to me, I wouldn't worry about anyone living paycheck-to-paycheck mistaking your technical looking homepage for a regular checking account.

One of your mentioned use cases doesn't sound right though - if I'm saving up towards a short-term spending goal like a car or house downpayment, I probably want a predictable balance and not exposure to sudden price shocks.