@JimKay 

Hi, welcome to the community.

The maximum connected clients for Archer MR550 should be 32 for 2.4Ghz and 32 for 5GHz. While usually to provide a great network performance, the recommended connected clients is 16 for each band. So ~50 2.4GHz smart home devices are a little overloaded.

And adding an Access point/TL-WPA7617 adapter for the current situation might help to increase the connected clients if you set another 2.4GHz SSID for the powerline network. However, I think it still can't reach 64 2.4GHz devices.

Have you tried to use Archer MR550 to handle the other subnet network devices(PCs/TVs/consoles) that could disperse connections to both 2.4Ghz and 5GHz bands?

Or "a more powerful one that would handle DHCP and the MR550 only the WAN access with 4G backup" sounds like a good idea, such as

Internet internet----MR550---- A bussiness router(such as ER605)+ Several Access points

Or Omada series also has 4G gateways that also support 4G Backup:https://www.omadanetworks.com/us/business-networking/omada/router/

Omada Networking Solution for Small to Mid-Sized Offices

Thank you very much.

Best regards.

  @JimKay 

Hi, in the lab test, Archer MR550 can support a maximum of 32 connected clients per band, which is primarily due to the CPU's data processing capacity and existing wireless interference. Adding an Access point doesn't alleviate the CPU pressure as  MR550 continues to function as the DHCP/NAT server, but it does help mitigate wireless interference, especially when using a different SSID and setting separate wireless channels. 
Furthermore, there are no specific experimental data available about the connected clients along with the increase in access points. More access points do not necessarily increase the number of client connections. One or two APs could be beneficial, but it is crucial to run field tests based on your wireless network environment to find an optimal value.

NX510v is a service provider model, and there is even less data about this version. But its wireless specification is quite similar to Archer NX600 whole lab test data is up to 128 for each band. There are other options, but overall ~150 devices(about 64 2.4GHz smart home devices) is a little overload for Home products. For your case, adding Deco Mesh products to create Ethernet Backhaul Mesh(WiFi Backhaul would not be recommended) might be another possible choice, for example Deco X90(2-3 packs), whose lab test data is also up to 128 for each band. 

The Omada range is a safe solution, as there are successful cases. (If you're interested in the Omada solution, you can start a new thread here Business Community > Routers.)

I understand you're looking for a suitable and cost-effective model for your network. However, I cannot guarantee that a specific model will function flawlessly. The lab test is conducted in a controlled network environment, whereas the actual home network environment is unique and has numerous uncontrollable variables.

Thanks for your time and patience.

Best regards.