They're "risk free".
They're "risk free".
But again, I might completely misunderstand things, but I don't think there is any investment that is absolutely risk free, which I guess is why you put it in quotation marks in the end.
As others pointed out, T-Bill indicates a shorter term than T-Note which is shorter than T-Bond... but either way, it's not that the instruments are hard to sell, it's a very liquid market, but the prices rapidly adjust so that yield to maturity for a given maturity date is broadly similar regardless of the initial yield and coupon.
It may be emotionally hard to sell and realize a loss if interest rates rise after your purchase, but it's not transactionally hard. If you are subject to a mark to maturity reporting regime, it may also look bad on your balance sheet, but I don't believe individuals would be subject to that.
For the rest of your money you can estimate when you'll need it and buy the appropriate T-bill, down to 4 weeks. Everything you say is correct, but only on that timescale.
The only risk is the US government collapsing, but in that eventuality you are very likely to be completely fucked no matter what you do.
Think of the duration as a lever, the longer the lever the larger the move in value that interest rate change make.
So they suffer from the same type of interest rate risk, but at order of magnitude lower impact.
https://www.treasurydirect.gov/marketable-securities/tips/
You could buy TIPS or I-bonds.
For example, failed states (even after complete destruction due to war), have a way of honoring primary residence property rights even after the rebuild of the country, many times including secondary properties that are not your primary residence (ie germany, poland after wwii, more examples exist). So if you know this history, you'd do well to make sure you own some property, not just "paper".
This is only one aspect of investing, but I've seen many people claim from the same source argument things like "holding REITs is better than a home because it's way more liquid and flexible", which... yes, but.