I think remote climate controls in a car are an ideal use-case for a simpler architecture.

Consider WhatsApp could do 2M TCP connections on a single server 13 years ago, and Ford sells about 2M cars per year. Basic controls like changing the climate can definitely fit in one TCP packet, and aren't sent frequently, so with some hand-waving, it would be reasonable to expect a single server to handle all remote controls for a manufacturer for all cars from some year model.

Or maybe you could use wifi-direct and bypass the need for a server.

Or a button on the key fob. Perhaps the app can talk to the key fob over NFC or Bluetooth? Local/non-internet controls will probably be more reliable off-grid... can't have a server outage if there are no servers.

I guess my point is if you take a step back, there are often simple, good solutions possible.