Do you think that Bingo may just be getting old? I imagine that just like humans and other animals the organ systems just don't work as well as they used to. If Bingo's liver and kidney function chem tests are normal that's a pretty good sign that he's not going into organ failure. Has Bingo had an abdominal ultrasound to look for masses inside that may be growing and pressing on things causing discomfort and digestive issues? If not, I would make a plan to take a couple days to make yet another trip into LA to somewhere that performs them. I know how much you dread a trip like that but people who live in the outskirts of cities do it all the time to see specialists and it sounds like youre tired of spinning your wheels. You could book an inexpensive hotel for one night (Van Nuys has a pet friendly place I just stayed at). Drive down early am for a vet appointment/evaluation and spend that night at the hotel with Bingo. Then early the next morning head out to the high desert in the reverse commute direction to miss traffic and you'd be home before 10am.
I just hope you can get some answers. Unfortunately many avian vets don't have access to the technology and expertise found in some major cities. My vet does his own imaging including ultrasounds but If I had to get my birds truly expert care like a surgery I would have to go to Angell Memorial in Boston. Not likely to do it for a budgie because they are so small that surgery isn't feasible but I would for a much larger, longer lived bird like Bingo.
My regular CAV recommended an ultrasound.
After much looking

I found a place that said they would/could do ultrasound (Culver City I think)
Made appointment.
Bingo’s Records were forwarded.
1 or two days before appointment. They called me up and said it needed to be a CT scan not ultrasound.
Quoted me a cost of $6,000.
I said no thanks.
This was two years ago….. I don’t remember exactly how long but it’s a sure thing it wouldn’t be cheaper now.
I’ll go along way for Bingo.
If that was the cost for a treatment with better than 50% chance of good outcome Iwould probably go for it.
But this is diagnostic only with no guarantee of finding the issue.
And .
No guarantee if they do find something it can be fixed.
Or how much it would cost to fix.
Lots of veterinary clinics say they do ultrasound but I haven’t found an any others that do birds.
The closest one my CAV knows of is in SF bay area.
Would be a 6 hour drive one way for me.