It may just be my training and flight experience (nothing implied to OP) but for me I would have instantly clicked to sports mode and punched up and away both in a combined effort would have cleared the other aircraft much faster than a descent by speed ratings of your aircraft being the lowest in descent. I would have been a good three to four times cleared than what you managed.
If these aircraft are routinely cruising at that altitude and that speed then punching up and away (No matter what direction your facing quick 180 Yaw, both sticks full up) seems to make more sense to me than your controlled steered descent toward yourself for it's safety. The FAA merely says: Always avoid manned aircraft, it doesn't stipulate down. YMMV and IMHO, and you would know best given situation.
Maybe something you would want to practice if this is a common thing on some flights in the future, perhaps a friend or spotter could just give you a verbal alert such as: "low aircraft inbound" to replicate your first hearing these things zipping around. That way your can get a feel for the switch position in memory and then just pump the sticks straight up if need be?
On a similar situation, you might just want to take the loss and set up for a CSC dump in the drink...would be much better than a mid air under some conditions. In your shown situation that aircraft had very little, if any cockpit. It would be a one in a million, but could you imagine that thing smacking him in the face given his speed? That would have been a death for sure.
