21AC is really high... Does the GI work?

For the other two voltages, since either the rectifier (which converts the AC to DC) is bad, or it's not getting the AC, so check that the AC sides of the corresponding rectifiers have voltage

As for the mpu, I'd guess from that description that it's getting four flashes or maybe five. Those bong sounds correspond to the led on the mpu flashing. If you watch the mpu you'll be able to get a clearer count. It should first flicker briefly, then start flashing. The first flicker doesn't count, not sure if sterns make a sound for that one or not. The amount of flashes you do get (minus the flicker) can help tell you what component on the board is bad

If it's actually 21VAC I assume all your gi lamps have blown. You might want to check that the rectifier board is correctly wired to the transformer.

It's possible the led is bad, or part of the other circuit is, since it's near the batteries too.

If the rectifier board doesn't have good voltages, none of the other boards will as they all come from there. I'd focus on getting your rectifier TPs correct as @setzcore says, before worrying about anything else. You may want to disconnect the rest of the boards from the rectifier board for safety while you work on it. It may be more worthwhile to just buy a replacement, but fixing it will be a good learning experience.

If the LED isn't even turning on on the MPU you're definitely going to want a corrosion repair kit (think big daddy sells them, or you can source your own parts).

Pinball life sells full brand new stern flipper mechs and all the parts in them.

The led only needs the 12V to work. And since you're getting the sounds you know the mpu is at least partially booting.

Since the mpu is getting to four or five, most of the stuff in the corrosion area has to actually be working, so you've got a good chance of getting it to boot, but that looks like it'll be a big job to clean it up for someone without good board repair experience. The number of bleeps suggests U11 or U12 is bad. I'd order one if each and replace them, see if you get more beeps. If that doesn't fix it, you could try replacing the socket for U11, but at that point if it still doesn't work I'd probably give up on it for now.

I've stuck allteks in multiple other stern games with the same sound board without issue. Can you take a picture?

The P1 display is going to need some major work, or may be a goner from the looks of it. Missing segment on P2 should be repairable. Take all displays out and reflow the solder on the connectors, then plug back in. See if the test button will get the game into test mode so you can get a better idea of what's wrong with the displays, solenoids, etc. Later Sterns can get some weird garbage in their memory in attact mode that can make the displays look like that, but it could also be an issue, display test should make it clear.

That means switch number 4 is closed. There may be more, but it only reports the lowest numbered

Check 12V as well as 5, and make sure to check them via tp on mpu

Try measuring across the cap with your meter on AC, general rule is if you measure over 1/4V you should replace it

I've rebuilt mine via Digikey in the past without issue. If you give me a list of what you can't find I'll check my records

Reflow probably don't fix a missing segment. The number itself is sent over the connector, not the individual segments

I wouldn't worry about it until you recap

I had that on my Meteor when I got it, caused the original board to do all sorts of weird things, and the Alltek either wouldn't boot at all, or wouldn't display anything. Not sure why they act different.

Check voltages on the other displays, each has test points.

Unplug all displays, test voltage on SDB, if it's good, plug in one display just in case, if it works, then should be safe to plug the rest in.

If you can get one with the same value, just jumper it in to test