A year+ (!) later, I thought I'd report back on this. The goal -- using a cheap tablet to access free wifi, and using phone as hotspot only for high-priority/sensitive stuff -- turned out to be do-able, but not great.
(1) A cheap tablet means a finicky touch screen (doesn't respond when you want it to -- responds too well to light accidental pressure). For example, those floating ads they've started using here are very hard to get rid of ack! And it can be hard to get streaming videos to start and stop when I want, so for example music might be blaring for several seconds while the screen is still blank so there's no way to press pause. All in all a small price to pay for free Internet, but not a seamless experience.
(Getting an expensive tablet would have defeated the purpose of this project; that's all explained above.)
(2) There's always something -- e.g. software upgrades, which you really can't skip -- to keep data use on the phone (the data I'm actually paying for) pretty high.
(3) Weird mystery xit can suddenly send phone data soaring. And when it does, it's very hard to backtrack what might have caused it. My daily average is about 1.5 GB (that's with almost zero streaming), and I suddenly had days last month with 13 GB and 9 GB. The only difference I could ID was that I had stayed signed into Facebook (not even having it open on a screen) (and I don't even have friends sending me cat videos etc). I've gotten religious, again, about keeping the wifi connection turned off except when in use, and avoiding FB, again, as much as humanly possible -- and frequently deleting unwanted cookies -- and data use has gone back to normal (knock wood).
(It just seems weird to me, in this high-tech age, that there /aren't/ better and more precise ways to track your data use -- all my phone could tell me is that it definitely was via the hotspot. I suspect it's a business choice by the tech companies, not a technical limitation.)
(4) I can't stream anything that requires a credit card -- or, I could, but only rarely -- so when it comes to watching movies and TV shows, I'm limited to free stuff or the occasional download. Fortunately there is a lot of good free stuff out there.
(5) It's awfully tempting to do non-sensitive stuff on the paid connection, because it's so much more convenient.
(6) It magnifies the effect of bad customer service (if you happen to run into that), because dealing with that can be quite a data hog.
(7) In one case I had to drive, fortunately only! about a three-hour round trip, to the nearest Apple Store to get something done that would normally be handled online. (Best Buy would have done it but would have charged big bucks.)
So it's not great, and I wouldn't recommend it as a long-term solution unless you really had to, but it's definitely do-able.
I found I definitely needed a keyboard for this, but not a mouse.
It /is/ nice to have something this small to grab and go for short road trips.
I kind of have to laugh at calling this "not great." How our expectations have changed from even a few years ago...
FWIW!