There are plenty of nice headphones with low impedance among grado, sennheiser, beyerdynamic etc, you kinda have to go out of your way to choose a high impedance “home stereo” style ‘phone

That’s funny since when I ordered a Beyerdynamic headphone on Amazon they sent me the high impedance model I hadn’t asked for.

Although the best one I own is a Stax which needs an entire speaker amp.

Wait, aren't Stax the electrostatic high-voltage headphones? I think at that point, "Convenience" is deeply crossed out as a selection criteria, and a portable phone while jogging is not a likely scenario :).

They actually made IEMs once. Beyond the technology being hard to manufacture, Stax's products are mainly just inaccessible because they have the traditional Japanese business approach of being run by 800 year olds who are actively offended by the idea that anyone they don't know might want one of their products.

their portable 'phones are fantastic, but yes, their full size tube amp is taking an inordinate amount of space on my desktop

https://www.headfonia.com/baby-stax/

"Nice headphones" is of course relative. I have two sets of Sennheiaer hd380. They plugged into anything and everything and sounded nice to my ears. They are super comfy. They still plug into my computer, synthesizer, and most of my stuff - but not of course my wife's iPhone.

(My old galaxy s2 had microsd, replaceable battery, grippy exterior,and of course 3.5 mm jack while being smaller in every dimension so I am skeptical as to some of the arguments other than "apple did it so entire industry will copy it")

As indicated above - I'm not am audiophile but I enjoy listening to music, and miss the easy convenience of 3.5mm jack on my phone along with everything else. A headphone amp would completely counter act the convenience and practicality.

Are you still using your Galaxy S2 or is it possible it wasn’t perfect despite managing to list all those things on a spec sheet at the same time?

It was perfect for the time. It's definitely lacking in features today, which is by definition true of any phone from that era.

I still have them both with me and in active use - mostly as remote controls, kids games, etc.

Perhaps more pertinently, my daily driver is a Galaxy Note 8, which also has a 3.5mm jack and MicroSD card. There's really nothing that my 4-generations newer, work-mandated iPhone has, in terms of functionality and definitely battery life, that Note 8 does not. iPhones are about sexy impracticality, and that's clearly where market has gone (Unlike S2 & S5, Note 8 also cannot be held without a case) - I'm just a old grouch who misses the olden days :).