Filter formula: Exclude data without a timestamp

=sort(   filter(     { Bryson!A4:S502; David!A4:S502; Justin!A4:S502; Michael!A4:S502; Manny!A4:S502; ScottC!A4:S502; ScottD!A4:S502; Zhenqian!A4:S502; Visitors!A4:S502 },    regexmatch({ Bryson!Q4:Q502; David!Q4:Q502; Justin!Q4:Q502; Michael!Q4:Q502; Manny!Q4:Q502; ScottC!Q4:Q502; ScottD!Q4:Q502; Zhenqian!Q4:Q502; Visitors!Q4:Q502 }, "Y"),    regexmatch({ Bryson!R4:R502; David!R4:R502; Justin!R4:R502; Michael!R4:R502; Manny!R4:R502; ScottC!R4:R502; ScottD!R4:R502; Zhenqian!R4:R502; Visitors!R4:R502 }, "")=FALSE,  ),   18, 1,6,1 )

Ideally, I want to filter out entries in A4:S502 on all these sheets that don't have a timestamp (Column R), but as long as R is not blank, that would be good enough, as I have an apps script do the timestamping and it should either be nothing, or a date. The first filter criteria for column Q works, but would need this second criteria for R.

My current formula throws the error "FILTER has mismatched range sizes. Expected row count: 4491. column count:1, Actual row count: 1, column count: 1.