Great firm, value rising every day, but the prospects for great short to medium term returns still don't seem to be particularly wonderful at the moment.

I ran across an article about "The Hidden Risk in “Religion” Stocks"
Old age and arthritis eventually catch up with “religion stocks.” No company can grow at a fast pace forever. Growth in earnings and sales eventually decelerates. That leads to a gradual deflation of the “religion” premium.

He mentions a number of these kinds of stocks, and says that "With the exception of Berkshire Hathaway, all of these companies have [crashed]."

investor.fm - The hidden risk in religion stocks

A number of people here have talked about exiting or reducing their BRK positions. Jim has been predicting very low returns over the next several years. I kind of like the idea of keeping long held positions in BRK in taxable accounts so that your inheritors get the stepped up basis, and exiting the holdings in IRA accounts.

I have had my eye on a mutual fund for some time, which I held for several years some 20 years ago but sold.
Its top 10 holdings are 57%, so those dominate the return. BRK is the #2 position at 9%.

So my idea was to sell my BRK.B in the IRAs and put that money into this fund. Then today, right at the market open I got a "S&P500 has hit a new high" notification. Hmmmm, maybe today isn't the best day to do the switch. How to decide when to do the switch?

Best day is when BRK is up and the fund is down. You don't know what the fund has done until after the market closes, which is too late. I don't think MFs report their NAV in realtime throughout the day.

A quick-and-dirty check would be to check the weighted gain of the top 10 (or 20) holdings and compare that with BRK's gain.

Well.
Anyway, just before the market close deadline, the weighted gain of the top 10 was 2.47% and BRK.B was 0.68%.

So, not today.