Yes, the limit-at option.
A very basic setup on a "blank" config with no existing queues or mangle rules might look something like this (do not use this blindly, just as example):
/ip firewall filter
print where action="fasttrack-connection"
;;; note the rules printed, and if more than one exist, what are their enabled and disabled status!
disable where action="fasttrack-connection"
print where action="fasttrack-connection"
;;; note all fasttrack rules should show as disabled now.
/ip firewall mangle
add chain=forward action=mark-packet protocol=tcp dst-port=IMPORTANT_PORT_1 in-interface=LAN_INTERFACE_HERE new-packet-mark=m-critical-traffic-out passthrough=no
add chain=forward action=mark-packet protocol=tcp dst-port=IMPORTANT_PORT_2 in-interface=LAN_INTERFACE_HERE new-packet-mark=m-critical-traffic-out passthrough=no
add chain=forward action=mark-packet protocol=tcp src-port=IMPORTANT_PORT_1 in-interface=WAN_INTERFACE_HERE new-packet-mark=m-critical-traffic-in passthrough=no
add chain=forward action=mark-packet protocol=tcp src-port=IMPORTANT_PORT_2 in-interface=WAN_INTERFACE_HERE new-packet-mark=m-critical-traffic-in passthrough=no
/queue tree
add max-limit=YOUR_UPLOAD_SPEED name=q-lan-to-wan parent=WAN_INTERFACE_HERE
add max-limit=YOUR_DOWNLOAD_SPEED name=q-wan-to-lan parent=LAN_INTERFACE_HERE
add limit-at=GUARANTEED_SPEED_RESERVE max-limit=YOUR_UPLOAD_SPEED name=q-critical-traffic-out packet-mark=m-critical-traffic-out parent=q-lan-to-wan queue=pcq-upload-default
add limit-at=GUARANTEED_SPEED_RESERVE max-limit=YOUR_DOWNLOAD_SPEED name=q-critical-traffic-in packet-mark=m-critical-traffic-in parent=q-wan-to-lan queue=pcq-download-default
add name=q-other-traffic-out packet-mark=no-mark parent=q-lan-to-wan queue=pcq-upload-default
add name=q-other-traffic-in packet-mark=no-mark parent=q-wan-to-lan queue=pcq-download-default
Once again, be warned that disabling fasttrack will increase CPU load.
If you'll run into problems, start with reenabling the fasttrack rule in firewall. This will break the queues but will also very quickly bring CPU load back to where it was.
Also maybe consider adding the limit-at= + max-limit= to the two "other traffic" queues, to make sure there's a bit of bandwidth left for it when the "critical traffic" suddenly tries to pump 500Mbit for one or another reason.