But I would like to provide higher priority for first subnet:
add dst=ether1-gtw name=Sq-Lan1 parent=Sq-Lan1-Lan2 priority=7/7 target=192.168.81.0/24
add dst=ether1-gtw name=Sq-Lan2 parent=Sq-Lan1-Lan2 priority=8/8 target=192.168.82.0/24
If you simply give higher priority to one of the targets you can end up with your 192.168.81.0/24 using all your bandwidth leaving your other subnet without any. The way I would prioritize is by defining 2 simple queues one for each target subnet. Allow max-limit 10M/20M on both queues so both subnets can have maximum speeds when not used. And last define limit-at parameters for each subnet. This will tell the mikrotik how to split the capacity in case of saturation. For examble you can set limit-at for 192.168.81.0/24 at 8M/16M and for 192.168.82.0/24 at 2M/4M. This way in case clients are using all the available bandwidth MKT knows how to split it.