Looking for a router suggestion for a network with 160 smart devices


New user here. Been reading this site and find it really interesting that I can use custom firmware on a router (this is news to me). There's a lot to take in so I'm hoping someone can give me some suggestions.

I'm currently using a Google wifi mesh network. I have four Google wifi mesh routers placed around my 2,000 square foot house. I have a house full of smart devices controlled by Alexa. Approx 160 devices including, light bulbs, power outlets, Ring security system sensors, August locks, cameras, etc., all Alexa controlled. I have a Synology NAS that serves Plex to a 4K TV over wifi. I installed all of this with very basic networking skills so I don't much about networking outside of port forwarding & setting static IP addresses, but I do like learning new things.

I occasionally get issues with slowdown streaming 4K videos from Plex to my TV that I would like to fix. I'm guessing it's all the congestion with the smart devices. I'm think that if I put all the Alexa devices on their own channel, and separate activities like streaming 4K videos and browsing from them, if might fix this problem. Maybe put Alexa stuff on 2.4ghz & everything else on 5ghz?

I'm also moving away from using Google services in general and getting into FOSS & open source software so I would be nice not have Google hardware running my network.

Does anyone have any suggestions to achieve what I'm trying to do? My first thought is I might need a couple devices bridged to get good coverage for all the smart devices, but maybe I can get away with a single router? My internet connection comes in from a corner of my house so it's not a central location.

Thanks for any help you can provide.