I want to be able to give a particular multicast stream higher priority from the rest of the traffic on a link so in case of congestion it would still get through without hiccups.
Everything I managed to find in the internet about prioritizing traffic on RouterOS deals with putting the traffic in queues:
1) Mark all traffic going on that link
2) Mark traffic that should be prioritized
3) Put all traffic for that link through a queue tree and make a sub-queue for the priority traffic and give that subqueue a higher priority.

But this method introduces unnecessary queuing and an increase in latency.

Does RouterOS have a simple prioritizing mechanism based on DSCP values? Something like:
1) Set a priority DSCP value to the priority multicast stream in /ip firewall mangle (prerouting)
2) RouterOS takes care of the rest and puts high priority packets on the outgoing interface first.

I's impossible to prioritize something without inspecting it.

It can, using the queues. DSCP is just a way to convey the information about required priority. Any operating system works the same way - classifies the traffic based on some local criteria or accepts external classification received in L3/DSCP or L2/CoS fields, and then uses a queueing mechanism to let higher priority packets overtake the lower priority ones while forwarding them.

... and max-limit should be 80-90% of the ISP's cap so you'll basically be wasting 10-20% of your bandwidth. Not good.