I don’t really have an recovery strategy in place, but what I do have is that all the smart stuff in my home can be controlled manually too. Light switches work just like dumb ones, thermostats have manual buttons and so on. So even if the server goes down I can still control everything manually. Obviously automations won’t work, but the house isn’t crippled if that single raspberry decides to go belly up.
It will never become an issue. As I mentioned, all the smart things I have can still be controlled manually. Sure, things like timing energy consumption to cheaper hours and turning on outside lights when it gets dark either stop working or needs to be manually controlled, but it would be more an annoyance than a issue.
And when planning for expansions I’m pretty strict that things stay that way. Everything has to work without HA, internet connectivity or anything at all besides obviously having electricity. Automations are just icing on the cake and they can save a few bucks here and there and offer quality of life functionality, but I’d never rely on those alone. Manual override has to be always an option.