A sequencer would be very low on my list to add-on to this synth, I mean how much time have you put in with the Matriarch's onboard sequencer so far? I was originally thinking "brand new to this synth" and the follow-up questions have me thinking "brand new to all this stuff in general." What do you find lacking or hindering when you sequence the Matriarch right now? I wouldn't buy another sequencer without being able to answer that question, and appreciate the fact that you're about to more than triple the total cost of this Moog set-up through filling 300HP of modules, so you should really hesitate to grab $600 or $1000+ modules that you're feeling wishy-washy on. There are a lot of extremely nice external sequencers in this price range that run circles around anything in eurorack.
No experience with the Metropolix. You could use voltage adders and switches to get chords going from a single sequence and quantizers may have this built in. That Sinfonion is built off the Harmonaig, which would be closer to "making sense" with the Matriarch if you must have your chords generated from a racked module and not the keyboard or something else. Just remember that the synth's keyboard isn't going to behave normally while you're sequencing the oscillators. Look at the Behringer 1027 as a much cheaper option.
I would stick with the more basic toolbox modules mentioned above and then see how you feel after a month or two. The Matriarch's magic is coming from the base component modules. You could go nuts with envelopes and mixers and companion VCO's and make it behave polyphonically if you wanted to but you'd be halfway to the price of a moog ONE with 1/4th of the voices. So really imo at this stage it's better to get some time with the synth and see what you keep reaching for while patching rather than to whiteboard it to death if you're submitting things like the Sinfonion as a question.