Alternator varying voltage

So I got a 2013 Tundra Limited back in late February/early March with 126,000 miles on it. Life was good, for a few weeks. In late April, I had arrived to service a pool. Was there for about 15-20 minutes, got in the truck to leave, turned the key and nothing. Then realized I had no dash lights, nothing. Got my voltmeter and put it on the battery. It showed 3.49 volts. I took the terminals off to clean them (although they looked ok) and while the terminals were off, checked voltage at the battery and it showed 10 volts. Put terminals back on and it showed 5.49 volts.
A couple of hours later my brother showed up to give me a jump. It took about 5 minutes with the cables attached to get enough to jump. When it started, rpms were about 400. Drove about 50 yards to the stop sign and the truck died. Jumped it again, had my brother remove the cables while I kept the rpms at about 800-1000, put it in drive and drove it about 8 miles to Autozone, shifting to neutral at stop lights and keeping the rpms up. When they went to check the battery and hooked up, the rpms went from the 400 back up to 600-700 and things checked out with the battery, they could not check the alternator because the battery did not have a full charge. I had read that a battery could have a bad cell, and it being a new truck to me, bought a new battery. When changed out, saw that the existing battery was from Nov 2023, so was basically new.
Things worked fine from then until two days ago, when the truck just shut off going 50mph. Was able to coast off a side road and into a driveway. Popped the hood, hooked up the voltmeter and it showed 10 volts. Took terminals off and it showed almost 11 volts. Some dude gave me a jump, which took a few minutes. Truck started and then shut off. We jumped it again and I had him remove the cables, while I kept the rpms up. Drove 22 miles home, the same way I did the first time, switching to neutral when needed to keep up rpms at stop signs etc.
Hooked up the voltmeter and saw that the alternator was only putting out 11.5 to 12 volts. I thought alternator. Picked one up and spent some frustrating hours replacing it. Initially when I started the truck the alternator was putting out 14 volts. I thought sweet, let it idle for a few minutes and the idle dropped to 400, voltmeter went down to 12.2 volts of alternator readings and then down to 11.8. I drove the truck about 4 miles to service a spa. When done and backing down the driveway onto the beach road, shifted from reverse to drive and truck stalled. Turned right back on, but with the low idle. Drove to autozone (about 2 miles away) and had them check. Battery was at 100%, but alternator failed voltage regulator test. When he disconnected the device, the rpms went right back to 600-700. I told him to check it again and this time it passed.
Does anyone have any insight on this? I did an ohms test from the positive side of the battery cable to the stud on the alternator and that checked out. I have not done the negative yet.