I don’t know why nay-sayers are freaking out about this aside from calling it a checking account. Most HSAs (like Health Equity which I use) have a threshold cash balance (say $500), and then allow you to invest the rest, even in equities. No one seems to yell at them for being irresponsible because a medical emergency may coincide with a market downturn.
Maybe this is inappropriate for a Launch HN, but do you see a path to profitability for this that isn’t based on selling advertising / marketing data? Is there sufficient income or a sufficiently large addressable market here to make this work on just cash management / tax planning?