As for whether to sell, I'm reminded of something my mom once said to me: you don't retire FROM something, you retire TO something else.
In the same way, it's not about the selling of some Berkshire, it's about what you want to do with that money, and when. Focus on the destination of that wealth, not its current location.

It sounds like you don't really have to squeeze your portfolio hard to eke out every last percentage point of returns, so sitting tight is always entirely acceptable. That's not my personality, I hate doing nothing (often to my detriment), but it has worked for you and for very good reasons. Depending on your situation, the reasonably generous current valuation multiples aren't, by themselves, a huge reason to sell.

Conversely, if you have something you want to DO with those funds, in a certain time period now or in future, then you want to do what's best to meet that goal at that time frame. If the time period is fairly soon, then yes, selling at today's "pretty darned good" valuation multiple makes a lot of sense.

If you're into a bit of arithmetic, have a look at these two posts.
mungofitch replies on buy of Berkshire Hathaway
mungofitch replies on buy of Berkshire Hathaway
Executive summary version: if valuations were typical of the last 15 years, a reasonable starting notion, the price would be around $368-383 these days, rising at the usual inflation + roughly 7%/year. Think of that as your target curve for the future price of Berkshire. And you can probably estimate the real after-inflation return from cash for a while: around 2.5% now, probably drifting down towards zero or perhaps a bit less. Given those two probable wealth trajectories, just figure out if your return from now until your target "use this wealth" date is higher with selling Berkshire today and holding cash, or continuing to hold Berkshire. For time frames within a couple of years, perhaps selling is the statistically better choice? And of course, if Berkshire dips, you can always buy back in.

As for the bonds question, short term paper seems the smart move at the moment. Positive real yields for once, and no risk of losses if you want to liquidate at any point because some better choice comes along. Rates are falling gently, but not nearly enough to tempt me into long term paper. Others may (will) have wildly differing opinions. I have T-bills maturing most months in the next year.

Jim