Nice, any tips on doing that for an entire home network? Or do you have to do it per device somehow

I use a a chrome extension for it. Otherwise when I hit their wall on another device or browser, I just skip it. Reddit seems fine with killing themselves off, I'm not going to any great length to mitigate their mistakes.

You can run your own DNS server locally. I run an unbound server on my router but you can also run dnsmasq on a raspberry pi or something. Pihole uses that. Then you can do whatever you want with local DNS (doesn't help with devices like Chromecast that ignore your local configs, for that you need your own router, which I also recommend. I use pfsense).

You'll need a local webserver running to serve the redirect, but that's a simple nginx config. Maybe I'll write a blog post for this stuff...

Can you do this without triggering SSL certificate validation issue?

Or do you have to manually accept your own SSL certificate (at which point you perhaps might as well install a redirect extension)?

There are browser plugins to redirect to old.reddit.com.