well first thing to note is that treasuries are paying good interest these days. like 4.5%. so a short term treasury ETF would make a lot of sense. they typically pay out the dividends monthly and in cash. so if you put the entire $1M into it you will be getting checks around $3,750 every month. totally free money (well, it will be taxed of course)
I would not keep everything in treasuries forever, long term they always lose to SP500. The market is hard to predict. Many people expect it to crash in the near term, so it might keep going up instead. You just never know. As someone said, dollar cost averaging over a longer period of time would make a lot more sense. So I would probably start slow, say, reinvesting that $3,750 check into an SP500 index ETF, just to see how it feels and to get more used to the idea. SP500 ETF is also going to pay dividends but only about 1.6% and it will be quarterly payment. That one should probably be reinvested back into the same fund.
In time, the usually recommended balance between bonds and stocks 40/60. But, again, that depends on many factors and does not work in all cases. If the fed keeps increasing the interest rates, it will be very hard for SP500 to compete. So right now, near term, I'd probably be like almost 100% in short term gov bonds. Later, when the interest rates fall and the market is done crashing I'd move into SP500 more aggressively. But still dollar cost averaging of course.
Now, while this is all very conservative and solid advice, it is also very boring. So to add some excitement, I would consider risking a small amount, maybe 2-3% of the total, but definitely never more than 5% and try higher risk, higher reward bets. Like some individual stocks maybe, or some crypto or anything that is volatile and has a great chance to go to zero. That money from the start should be considered lost. Which it very well may be. But it would certainly make investing more interesting and you will learn a lot of things. So I would consider it more of a downpayment for education. As long as you are strong and sure of yourself and never gamble more than a small, fixed fraction of the total (and never trade on margin, never short, etc) then it's really not a bad idea. Like a voluntary tax on the other very safe bets described above. And then hey, maybe that stock that you pick will go to the moon, or the crypto sh*t coin will go 100x against all reason. Not likely, but not giving yourself a chance is not right either.