I think the trick is reducing whipsaws but avoiding large parts of big downtrends.

The trick to reduce whipsaws is to have the sell signal be a few percentage points below the buy signal. But not so much below that it keeps you in longer when encountering a big downtrend.

The problem with using exactly the SMA crossing is that you'll get chatter when the price hangs out near the SMA, as it crosses over back and forth with each little wiggle.

Anywhere from 1% below the SMA to 4% below work fine.

Another way would be to require it to be below the SMA for few consecutive days or weeks. I used 4 consecutive weeks at one time, but now find that 4% below works well and is simple.


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Returns and risk metrics likely improve considerably with bond allocation when "bearish"?

The whole purpose of getting out is to step aside from the deep downturns. Not to make a gain while you are out. You don't want to be in bonds because that's an investment, not a safe haven. Investment (bonds) can go up and down. We want something that does NOT go down. If if goes up a little bit, that's just a happy bonus.
What you want to do is put the money into a very short-term ETF or MM fund. ICSH, BIL, SGOV, etc.