The other added benefit is that all my other devices already have my AP's wifi creds and will connect to it automatically.

I use my Samsung S10 for exactly this as it has multiple radios that allows connecting to wifi and hotspotting to share that connection with other devices. Great for Chromecasting.

Also works well if your 2nd device doesn’t support captive portals (Chromecast, Tesla car).

It's quite frustrating as it's ideal on paper but they fall on the last mile to make it practically open.

It really is. Where I live when you run into the limit of your data package, your network is usually throttled to 100kbps. I changed my plan to just 3GB per month because I was staying at home most of the time due to the pandemic. Now I'm pretty much back to my old routines, but I didn't change my plan yet. I have a 45 minute train commute and 3GB can be used up in a few days just browsing reddit and loading news sites.

Anyway, google search, hacker news and facetime audio work as normal at 100kbps. Google maps works with a bit of patience. Virtually nothing else will load. 5 years ago most text-based things worked at this speed albeit slowly. Now everything is so bloated and so much content will not load show until fonts and things are loaded.

The connection was pretty damn good, considering I was somewhere over the Atlantic. It was shocking to me how much more enjoyable the flight was, makes me wonder how hooked I am to being connected. (I also had extra legroom and an empty seat next to me though.)