Michael, You're welcome.
You should contact your lung guy and have a PFT Spirogram with DLCO. Make sure it includes a 6 min walking test. If you haven't done so already. If you need or want anything else, just let me know & I'll try to give you the answer.

I see Inogen has a Gen 5 machine now with a much longer run time.

There are also machines made in India that run about half the price of the Inogen. Nothing against them, but, I figure if I had as much trouble with the Inogen and it's rated one of the best - then why would I expect a lesser machine to be worthwhile? Maybe if it was a whole lot less.

The absolute worst part of the whole thing is how badly they rake you over for the batteries. I know they are medical equipment, but, come on,,,$600 for a battery?