If you shop around, you can get some decent T-Bill equivalent yields in simple funds or ETF's that are hands off without a lot of ongoing weekly maintenance.

This is absolutely true. Up to about 5 years ago, I used funds for my cash and never bought T-bills individually. BUT, what I found was that invariably, the fund I chose would begin to flag and then other funds had better yields, or T-bills had better yields. It didn't bother me so much when my balances were relatively low, but now that I am retired, ALL my current spending comes from interest, dividends, and capital gains, so I really, and literally, can use any of the extra yield. So the choice is to "chase" funds for the better yield, or simply buy the "source material" directly (T-bills).

I'm getting 4.24% right now in SCOXX

The minimum in SCOXX is $1M, so on $1M, a difference of 0.11% (my T-bills are now about 4.33%) comes to $1,100 a year. That's $1,100 more (minutes taxes) that I can spend.

I also have a chunk of cash in FZEXX

I should probably look into tax exempt money markets a bit. I think I was in the 32% bracket in 2024, add the 3.8% NIIT to that and the effective rate is 35.8% (I think some tax exempt interest isn't subject to NIIT, but I'm not sure which). I'm also in a no income tax state.

Right now FZEXX has a yield of 2.91%, divide that by (1 - 0.358) and you get an equivalent yield of 4.53%. That's higher than the T-bill yield of 4.33% so it ought to be considered. However, I plan on 2025 being a lower income year (this is a year I am trying to take only short-term capital gains and balance them against al sorts of old capital losses that remain to be taken), so I may be able to stay in the 24%, or mostly stay in it, so the decision can wait till the end of 2025.