Investing inside an HSA

I don't think the $2,000 threshold is tied to any sort of federal regulation. In other words, it's specific to your HSA plan. You'll have to refer to their documentation regarding whether you can continue to invest if your balance dips below $2,000.

I expect that you will have two separate accounts. One would be an HSA account into which you and your employer make contributions. The other would be your actual investment account, through someone like TDAmeritrade. You transfer funds from your HSA account into your investment account when you want to invest them. If you have a medical expenditure in excess of your HSA account's cash balance, you would have to transfer cash from your investment account back into your HSA account or else pay out of pocket.

As that may require you to sell securities, liquidity and volatility should be primary considerations regarding which investment choices you make. I'm currently invested in a number of low volatility, high dividend yielding stocks in my HSA investment account, but even that may be too aggressive for the average person.

This is correct, but I want to clarify that this investment account is opened through your HSA provider, not independently.

I’m able to have automatic transfers into my HSA TDAmeriteade account that I then invest into a total market index ETF that has no trading fee. Used to be VTI, but I had to change it since that’s not free anymore. Low fees allow a great set it and forget it approach.