Using this as a guide is a starting place. You will set up a Queue Tree with a Parent Queue (this is for your over bandwidth) and then Children Queues (one each for the different user types)
We are going to start on step 2.
/queue type
add kind=pcq name=Download pcq-classifier=dst-address pcq-rate=120M
add kind=pcq name=Upload pcq-classifier=dst-address pcq-rate=120M
Next, we need to add the mangle rules. Order matters. First Packet Mark the dedicated user. Then add a catch all for everyone else. Change "192.168.88.5" to your dedicated devices IP or use address list instead. I use chain=forward to exclude inter LAN traffic and the Routers traffic.
/ip firewall mangle
add action=mark-packet chain=forward dst-address=192.168.88.5 in-interface-list=WAN new-packet-mark=Dedicated_Download passthrough=no
add action=mark-packet chain=forward in-interface-list=LAN new-packet-mark=Dedicated_Upload passthrough=no src-address=192.168.88.5
add action=mark-packet chain=forward dst-address=192.168.88.0/24 in-interface-list=WAN new-packet-mark=General_Downlaod passthrough=no
add action=mark-packet chain=forward in-interface-list=LAN new-packet-mark=General_Upload passthrough=no src-address=192.168.88.0/24
Now setup the Queue Trees. You will have both a Download and Upload for each Parent and Child Queue. Also set a higher priority for you Dedicated Queue.
/queue tree
add name=Download parent=global queue=Download
add name=Upload parent=global queue=Upload
add limit-at=15M max-limit=120M name=Dedicated_Download packet-mark=Dedicated_Download parent=Download priority=1 queue=Download
add limit-at=128k max-limit=120M name=General_Down packet-mark=General_Downlaod parent=Download queue=Download
add limit-at=15M max-limit=120M name=Dedicated_Upload packet-mark=Dedicated_Upload parent=Upload priority=1 queue=pcq-upload-default
add limit-at=128k max-limit=120M name=General packet-mark=General_Upload parent=Upload queue=Upload
Now your dedicated device should receive no less than 15M/15M and be able to use the whole 120M/120M when available. And the General Users will share the 120M/120M or what is left of it equally.