Just remember we have been running web sites for past 30 years. Actual computing power to serve something static is minimal in current day.

For a static blog? The limiting thing likely is the outgoing connection... Would not be surprised if you could saturate a 10 gig link if the USB link allows

If you compile it to a static site with something like Jekyll, I expect an old phone could handle pretty much any single blog in the world. And of course if you ever reach astronomical traffic, you can always add a CDN.

If you’re only serving static content, I’d expect the home router to be more of a bottleneck than the phone itself.

Why not. Put it behind cloudflare and you are good to go. Even if you don't put it behind cloudflare you should be good. There are many entry level vps that are not nearly as powerful as a podern smartphone, and people host plugin heavy WordPress on then.