I have fairly new pads on my mtbike. Hydro Shim XTR brakes -- whatever the good ones are. I bought the pads cheap from Amazon. They were fine for a dozen hard rides last season. I took the bike out for first time this spring and the fronts squawk deafeningly. The rears not much better. I used fine sandpaper on the disks and took the pads out and cleaned w rubbing alcohol then sanded them all. (The front pads seemed to have more black residue as I cleaned and sanded.) Tried again: exact same, no improvement. People say to sprint fast then hit the pads hard to bed them in. Well I don't want the neighborhood to hate me. I've tried this extreme-scream bedding idea like 5 times. I'm done! They are not getting any better. I just ordered fresh pads. Last year that was the only thing that got the brakes to quiet down as well. I AM NEW TO DISK BRAKES!

I would never use cheap pads on anything, that is the best way to poor braking. Amazon is not a bike shop so whatever they are selling I wouldn't buy for a bike unless beyond desperate but if I have XTR brakes that is not a problem. Get good KoolStop or SwissStop pads get good rotors and drag the brakes down a hill a few times.

If you have dirty pads and rotors sanding could help but I would soak them in alcohol and then set them ablaze in a safe environment and that can help sometimes. Usually it works but sometimes they are so contaminated it won't burn out no matter how many times I do it and sand.

Did you use rubbing alcohol, which can contain oily additives, or 99.9% isopropyl alcohol? Acetone or actual brake cleaner also work well. After you degrease the friction surfaces do not touch them.
If you're going to sand pads place the abrasive paper on a properly flat surface to keep the pad flat; it's considered good practice to move the workpiece in a figure 8 using light even pressure to keep material removal even. Be sure that abrasive particles aren't left embedded in the surface when you're done (good abrasive paper should minimise this, don't use the stuff made for woodworking..

Manufacturers often publish recommended procedures for bedding-in, and for a good reason. AI suggests:

1. Low-Speed Cycle: Ride the bike at a moderate jogging pace. Firmly and evenly apply the brakes to slow down to a walking pace. Repeat this 20 times per brake.
2. High-Speed Cycle: Accelerate to a sprinting speed and apply the brakes very firmly until you are at walking speed, taking care not to lock the wheels. Repeat this 10 times per brake.
3. Cool Down: Ride the bike without using the brakes for several minutes to allow the rotor temperature to cool and for the pad material to set.

Cheap pads are a false economy, buy original equipment or big-name after-market. If you change to different pads you should do the bedding-in again.

I am curious about the new pads. Are they the cheap, unknown brand, again. Putting lousy pads on any brake caliper is a good way to make the caliper lousy.

Cheap is bad? Per my report they were great all last year. They got the squawk from disuse over winter. Cheap pads maybe have something in them that rusts, that better pads don't have?

If you live anywhere your bike storage needs heating during the colder months, it's entirely likely the makeup of those cheap pads had them drying out over winter and disuse. If they were softish when new but harder now that could make 'em squeal during use.

"Great all last year", " dozen hard rides last seaon",
A dozen rides in one season is not a test of quality or durability. A couple of years with many dozen rides might qualify. 12 rides and they become crap, despite not being used, indicates they are crap.
Of course, it could all be partially, or entirely, user error.
I have mid priced Shimano rotors and pads on my only disc brake bike. They have 4 years and at least 6000 miles of use. It is a road bike, so braking is not as extreme as off road. And, they have never squawked.

If cleaning and sanding the pads (and rotors) didn't do anything, you need new pads.

What I genuinely don't understand is why you put the worst of the worst cheap pads on the best of the best expensive calipers? The parts work together, one is only as good at the other. If you have amazing calipers but cheap pads, you've got a cheap brake system.

Quality pads will last a long time, both for use and for storage. That being said, any pad which gets contaminated with brake fluid, grease, oil or lube is junk, toss them and get replacements.