Yeah you cannot use RTCP because it is hop to hop. ibc mentioned it in Forwarding NTP timestamp from source - mediasoup libraries - mediasoup
mediasoup does not relay RTCP packets, it consumes/terminates and produces its own ones as any proper SFU. RTCP protocol is hop by hop, not end to end. This is how things are and how things must be.
You can only get delay by one hop. I personally used RTP extension abs-capture-time to get the required delay