MMFUSDT
MMF / Tether USD
crypto

Read the bogleheads wiki. It covers things nicely.
You have a few things to get through to them:
(1) Hourly fee only advisors who are fiduciaries only. An AUM advisor will cost you 30% of your lifetime gains, because you lose out of the returns on their fees.
(2) Low cost index funds provide the best diversification. You're talking about a mix of VT, a bond index fund, and cash in a HYSA or MMF.
(3) At their age their primary criterion should be risk tolerance and goals. If they never want the money, put it in VT and let their kids get the step up basis in the will. If they're saving for fun expenses, they can afford more risk (i.e a large % in VT). If they're saving for necessary expenses (e.g., home repairs or retirement home) they should have more in cash& bonds.

Arbitrage & MMF fund both are fine for goals between 1-3 years. I use ABSL MMF & Parag Parikh Arbitrage Fund for my recurring annual goals & emergency buffers as well. I am fine with it because I don't mind taking a bit more risk and also my recurring annual expenses are not fixed at 1 year its more like 1.2 to 1.5 years. So it varies person to person.

Hey again, I've just started using arbitrage for recurring annual goals but said MMF is for 1 to 3 years and yet you use it for annual goals? Nothing wrong in it though. I mostly went with Arbitrage for tax efficiency as I plan to redeem 1 years old units in a recurring manner.

MMF

Im not knowledgable about cash plus and how it interacts with Vanguard MMF.
But VUSXX was yielding 3.72% (7-day SEC yield) last I looked.

I am trying to identify a non core MMF to use to hold cash in an overdraft protection setup. I assume that FDRXX is an eligible fund based on my poking around.
Do I simply buy this fund as I would any other MF? I also assume that I will be able to replenish this cash balance in the CMA on an as-needed basis and every so often create new buy orders.
Any other gotchas that I need to be wary of? Also, can I put this fund into a brokerage account (I have a couple extra ones lying around) or am I constrained to a CMA?

The Cash Plus account doesn't need to exist. You'd be very off in USFR/TFLO during rising rate environments like the present, and VBIL/SGOV in falling rate environments.
Unlike bank deposit products, these are exempt from state income taxes, too, which many people forget to discount these bank deposit products by for a fair comparison.
You can see the current compound rates of all the popular MMFs and MMF-like ETFs in the MMF Yields tab of my [rebalance calculator](https://invest.mcawesome.org/).

The settling time part and requirement to manually input the trades are, as the original commenter put it, a minor annoyance.
Schwab's real weakness is not having a proper default core position that bears interest (like Fidelity and others). As another commenter said, they make money off peoples' laziness or lack of understanding that to get any decent interest rate from Schwab you have to buy MMFs manually. It's pretty scummy and is defensible enough when everybody operates that way, but they are now in the minority in having garbage bank sweep rates.
And it can create real headaches when your MMF position isn't treated as cash & has to be held for 30+ days to be marginable or usable as collateral for selling puts. This makes it basically impossible to get interest on new premiums in any put-selling strategy, which simply happens by default at a lot of other brokers.
Not sure why you are so dismissive of things that inconvenience other people just because they clearly don't affect you.

Pretty sure those are still subject to settling times and require margin to bypass settling, which can really eat into your dividends if you don’t hold the MMF for long and want quick access to the funds for trading.
And, at least in the past, things like SWVXX had to have been held for 30+ days to become marginable, which is just ridiculous for people trying to do things like sell puts.
Schwab is basically worst-in-class IMO when it comes to handling cash investments. Hopefully that changes soon.

First rule of windfalls: first, do nothing. Since you know your windfall date a month early you can’t do anything but plan, so you have successfully accomplished this step already. Congratulations.
Forget the midterms; either something will happen or it won’t. Either that will work in your favor or not. There may be an October surprise before your payout date, or everything will change after the election, or it will be more of the same. We’re overdue for a correction, which doesn’t mean a correction is on the horizon. Nothing you can do about that.
The next big decision is lump sum vs DCA. In the long run, lump sum wins about 2/3rds of the time (iirc). Which at 75 is not anything you should care about; you aren’t trying to win, you’re trying to not lose. Preservation and income are your goal, not max growth. Plus you are risk averse. DCA into a conservative allocation is probably the right choice for you.
Conservative means bonds for stability and equities for inflation. So the third decision is your target allocation at the end of the DCA, and how quickly you will get there. Decide where to hold the funds in the meantime. I’d go with a MMF but an HYSA or something like SGOV is not wrong. Anything safe and liquid with a yield.
For the bonds, decide on funds vs nominals vs a mix. I prefer funds (reddit hates funds), but a ladder for at least the first few years will provide greater stability if that is a priority. Bonds are getting weird lately.
You want to avoid making a bad decision based on current events. The way to do that is to make a plan, write it down, and execute. You can even include contingencies - if X% market crash, then plan B. That still counts as staying the course because you made that decision with a level head, not while actively panicking.