Router: https://www.turris.com/en/omnia/overview/
Media Center: https://osmc.tv/vero/
Cloud (Use TrueNAS Scale): https://www.truenas.com/systems-overview/
Phone: https://www.pine64.org/pinephone/
Watch: https://pine64.com/product/pinetime-smartwatch-sealed/
Smart Thermostat: https://hestiapi.com/product/hestiapi-touch-one-free-shippin...
If you go this route all devices will be running Linux. The one OS route is kindof nice, hence the pinephone over open android alternatives (like Graphene OS).
I sorted from least to most technical. I also tried to pick the least technical challenging in each category. The Dell stuff should just work. The Phone will require some tinkering for the moment.