We successfully turned a high-card hand into a multi-street bluff before checking back a rivered pair for showdown value.
Checking is the preferred play here. While we have the range advantage, A6d has decent showdown value as high card and doesn't benefit much from folding out worse air.
Note: Betting is a minor mistake; this hand functions better as a check-back to realize equity and protect our checking range.
The Queen is a great card for our range, and A6d is a perfect bluff candidate because we have zero showdown value but block some of BB's continuing range.
Checking back is mandatory after rivering a pair. We now have enough value to beat BB's missed draws (like spades or 53s) but aren't strong enough to get called by worse.