Have you tried to check up on the shop’s claim that all they can get is AC Delco? Its obviously not the same vehicle but I had no trouble getting after market replacements for my Oldsmobile. I think they were Monroes or something - all of $75 for a pair and almost as easy to put in as it is to change a tire.
Aside from that I’m not buying the other part of the claim that they break frequently. I would buy a modified story about that - something like once the air system gets this old they just continually cause trouble for one reason or another (e.g. soon after the shocks, the sensor goes bad, then the compressor goes next, then the air lines, wiring, whatever). Then I can see a shop being wary.
So in the long run you might be happier just getting off of the air system - there are lots of things that can break on it. I can’t comment on the ride quality or any of that though b/c I just don’t know.
But I can’t imagine its that big a deal to just toss new air shocks on it.