To get more control over what the modem/router ahead of the MX actually does, I have ordered a Draytek 2862-K. It handles IPv4 and IPv6 natively, and has a lot of flexibility with how NAT'ing is handled, a RADIUS server, and, at a price, there is a version with an inbuilt LTE modem. Be aware that Meraki does not fully hanle IPv6 yet; for example, some analytics will be missing.
I need this because the MX does not pass Source Specific Multicast correctly, and the Draytek does. But the MS switch in its default configuration does handle it, so I can just bypass the MX from the router to the switch and thence to the Playout device.
I have replaced my entire network stack three times this year. All the suppliers are well known Bay Area Corporations. It seems they just don't "get" how important it is to handle the TV feeds from the content providers. So instead of switching yet again, I've looked for ways around problems.