From all my research on this I came up with 3 common things.

1. There's a low hanging plug on BOTH sides of the cabin behind the corner kick panel. Its bolted together with a 10mm head bolt, unbolt those, spray it out with CRC electrical cleaner or similar, blow out with compressed air. This should take care of your alarm issues since these plugs are related to body control. This also fixed my wiper, fuel level, and interior lighting stuck on issues which would explain a battery drain. Then put the closet hanger desiccant bags and bucket of RV desiccant in there with fans to move the air and close it up. Let it dry out in the sun.

2. Is the no start thing intermittent? Mine would no crank, no crank, then start after a few tries. It stopped doing it after a few weeks and has only done it once since then (4 months since getting wet). I've also heard of a GX470 having the same issue and he ended up replacing the starter and solenoid assembly with no issues since. We both believe it's a wet/corroded starter solenoid causing the intermittent no crank no start. Which doesn't really make sense if it stayed dry up in the V of the engine.

3. ECM. After drying everything out, cleaning connectors, and maybe replacing the starter (or just the solenoid) do you have any ECM fault codes? I would shy away from ECM replacement unless I had an ECM internal error fault code telling me to replace it. I've read of others saying it fixed their issue but I wonder if it was moreso just the cleaning of the corroded connections to the ECM from humidity in the cabin -> see cleaning connectors and desiccant steps above.

Yes the new ECM will most likely require Toyota reprogramming many things. Good luck!