@chriswilcox
I think the best choice for your Deco mesh upgrade would be Deco X90. According to recent CNET article I can't put link to in this post, Deco X90 should be released this month. Here is relevant paragraph from the article:
"This month [January 2021] will mark the launch of the Deco X90, an AX6600 mesh router with a tri-band design featuring a 2.4GHz band and two 5GHz bands, one of which you'll use as a dedicated backhaul between the router and its satellite devices. That one will sell in a two-pack with the router and one satellite for an asking price of $550."
Article title: "TP-Link's new router lineup at CES 2021 includes Wi-Fi 6E mesh and built-in Alexa"
So, if you could wait another 2-3 months, you may be able to get best of two worlds: dedicated backhaul and WiFi 6 support.
If you can't wait, for your environment I would suggest going with X60. Your WiFi speeds will be at least as fast as what you have now, and you will future proof your home network with WiFi 6 support.
I run Deco mesh which consists of two M9, Main and wireless Satellite, and wired M5. Don't get me wrong: I love Deco M9, but I've chosen them with different requirements in mind. I needed form factor that I can easily wall mount. I don't expect to benefit from WiFi 6 in the next 5 years. My ISP provides 300Mbps and I won't upgrade to anything over 500Mbps any time soon, no need.
Also, my WiFi network utilization is light. Heavy hitters, such as servers, desktops, even TVs - all wired. At any given time I have under 10 mobile devices on my WiFi, and half of those are (almost) zero traffic IoT devices. No WiFi connected security cameras streaming constantly.
Yet, even with that light load Deco M9s are warm on touch. Not hot, but definitely warmer than room temperature. Which comes to important point: heat dissipation. If I were to deploy mesh for medium to heavy WiFi load, I would definitely go with mini-tower node form factor. Mesh node with M9 form factor might overheat much sooner than mini-tower.
Also, think about mesh node hardware design. With mini-tower node, there are less sacrifices in design. Larger antennas can be placed inside, faster CPU can be used without concern for overheating. Overall, less performance sacrifices required than when trying to squeeze all hardware in M9 form factor.
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A few housekeeping advises for your current Deco mesh. I would have expected to see faster than 500Mbps from M4. If you run Deco mesh in Router mode, check TP-Link article Using QoS feature of Deco. You may wish to increase value of QoS parameter(s) to 1000 to see if it improves WiFi speed.
Also, if you have 2.4GHz enabled in Deco app WiFi settings, disable it temporarily for the purpose of testing and run speed test for device connected to Main Deco M4 with guaranteed use of 5GHz.