A password manager because if anything goes wrong, you’ll be completely screwed.
What you SHOULD absolutely self host though is a password manager, so you can be in control of your most sensitive data.
Regarding email, I think everyone should absolutely self host it, but it’s less and less viable in this google/Microsoft duopoly world. But ideally everyone would self host it. The reason why people advise against it really comes down to lack of real competition, and the two tech giants dictating how we violate every RFC possible.
Just…set up two RPIs with Pihole instead of one? Chances are your router can have a fallback DNS. Sure, you have to update the rules in both places, but honestly it’s not a big deal, and you now have redudancy.
I’m running 2 powerdns recursors and authoritative servers, and 2 piholes (long story why so many), and none of them have failed on me so far, and when I took one of them offline, I didn’t notice anything because the other took over. And if anything REALLY fails, I’ll just switch my router back to using cloudflare or Google or quad9 temporarily, and at least Internet access will be restored so people can at least browse the internet.
Pihole also has an API and a home Assistant integration, so you can create an AdBlock toggle switch for others, in case it blocks something and they need immediate access. Not ideal, but it’s a doable workaround.
This is really something that’s super easy to self host, and mitigate if something goes wrong. Especially since that commercial router is already a single point of failure for most households.