so you want the router get to know that some link is “slow”? it’s impossible - there’s no even packet drop, according to your words
in ‘-xor’ mode, the link is selected for each ‘stream’ (based on bonding settings), it do not change during transmit
it’s like in situation with download QoS on the router - your router can’t know whether it’s ISP’s non-guaranteed link’s congestion or your users just don’t use Internet =)