Expanding on @ldir's idea, I would make a copy of simple.qos:
cp /usr/lib/sqm/simple.qos /usr/lib/sqm/simple4twitch.qos
cp /usr/lib/sqm/simple.qos.help /usr/lib/sqm/simple4twitch.qos.help
That way you have your own qos-script which will still show up in the SQM GUI.
Then I would make the following changes:
PRIO_RATE=
expr $CEIL / 3 # Ceiling for prioirty
to PRIO_RATE=
3500 # Ceiling for prioirty
To make room in the highest priority bin for your twitch streams.
Next step is to elevate all packets from the twitch stream to the high priority class. Since I have no clue about how to unambiguously identify twitch-stream packets, I would probably try to mark these by IP address. So the first step would be to make sure the twitch machine gets a predictable IP address, by giving it a static lease in the Network -> DHCP and DNS -> Static Leases sub-tab.
Next step conceptually is to add filters that will move all packets from the static IP-address (in the example 192.168.1.111) to the high priority bin by adding something like tho following to the end of the egress section of simple4twitch.qos:
$TC filter add dev $IFACE parent 1:0 protocol ip prio 10 \
u32 match ip dst 192.168.1.111/32 0xff flowid 1:11
$TC filter add dev $IFACE parent 1:0 protocol ipv6 prio 11 \
u32 match ip dst 192.168.1.111/32 0xff flowid 1:11
Mind you, I have not tested that, I just composed it in the discourse...
An alternative, if you stream alternating from different computers, would be to still change the bandwidth of the high priority bin, but instead of using a static lease, make sure all your twitch packets are DSCP marked as EF. Then use New-NetQosPolicy to make the twitch application (lets assume twitch.exe) emit its packets marked as EF:
New-NetQosPolicy -Name "twitch" -AppPathNameMatchCondition "twitch.exe" -PolicyStore ActiveStore -NetworkProfile All -DSCPAction 46
Use Get-NetQosPolicy -Store ActiveStore
to list existing policies and Remove-NetQosPolicy -Name "twitch"
to remove policy if you do not need them anymore.
IMHO the second option probably is easiest, as all you need to do is to fudge the bandwidth assignment to the high priority class.
That said, there is a reason why traditionally the higher priority classes get less bandwidth than the lower ones, because otherwise everybody will just use the high priority bin and you are back to square one. In your case that should e easy to avoid.