Seems to be the question is, WHY would you do covered writes on illiquid brkb vs very liquid SPY , in size ? The correct comparison should be an spy covered write strategy vs a brkb covered write strategy.

I don't have the numbers in front of me at the moment, but...
When I calculate what Jim calls "the implied interest rate", it is generally lower for a BRK call than the similar SPY call.
That's why BRK over SPY.

Also, you can pick your spots better with BRK than SPY. Price/book low means the odds are in your favor, price/book high means the odds are against you. I don't think there is an equivalent easy to see thing for SPY.

Also, in these deep ITM long dated call options the liquidity doesn't really matter. The counterparty is not other investors. The counterparty is a computer running an algorithm.
You run the numbers to decide which stike & date you want, then look at the bid&ask numbers to decide what to offer. If that doesn't meet your acceptance criteria you don't even place the trade. If your offer is close enough to the ask, you will get an immediate fill.

(Jim has said that you can usually get a fill at 3/4 inside the spread.)

There is very little action on these calls. There have been many times when my trade of 1-4 calls has been the ONLY trade for many months. There have been many times when my trade was the very first time that particular option has ever traded. Liquidity doesn't come into play.

Of course, I could be all wet here.