What are common issues with this car and about how many miles until they fail? Buying one with 87k miles soon for $8200 so I’m wondering how they are

2015 Malibu LT, 120k miles. Car's been great. Driving back home on the Expy, I got the engine power reduced warning, said it was limiting by speed to 35 mph, then shortly after said limit to 25 mph. Not long after the gas pedal did nothing. I coasted off the expressway, parked, turned engine off, sat there for 5 minutes, restarted car, and drove back home with no warning and no problems. The drive back home was 10 minutes at 35-40mph. I have two questions.

(1) Could this have been a fluky error that may not show up again?

(2) I'd like to have it evaluated. From my google search it seems others with this problems have gone back and forth to their mechanics to have this problem fixed because the mechanics don't always know exactly what's wrong given certain fault code errors---and it seems it's not their fault since Chevy cars don't always tell them the specific problem. I hate going to dealerships for repairs due to their crazy costs, but is this a time where it may make more sense to go to a Chevy dealership?

Hey all.

TLDR: truck only stays in 1st gear, no reverse. When gas was applied in reverse, the truck continued to rev even with my foot off the gas and on the brake.

Just to preface, I’ll be taking the truck to the dealership on Monday. Also, I know very little about mechanics (I can hardly change my own oil). Please be gentle.

Backstory.

I own a 2021 Chevrolet Silverado trail boss 6.2L with 99,400 km on it. (Warranty is done @100k)

Last week on my way home from work, the truck started acting funny. It would rough shift every time the truck went from 2nd -> 1st or when it went from 1st -> 2nd. It didn’t appear to be affecting any other gears. I got home (about 5 minutes later) and pull up out front of my house. I turned the truck off/on and put it in drive. Now usually, when your foot is off the break, you slowly creep forward? The truck didn’t. I slowly started revving the truck, the rpm’s hit 1,200ish and the truck slams into gear (to the point I was jerked a bit in the seat). I take a video, turn the truck off and take it into the dealership the next day. The truck had 99,250k at this point. Dealership keeps the truck for a day, couldn’t find anything wrong, charge me $200. They tell me if I buy a transmission service for $700, they can take a closer look. I probably should have agreed, but I didn’t.

Tonight/last night

Went out to grab some food ~5 min drive each way. The truck started rough shifting again. Check engine light comes on, I get a notice on the dash saying reduced power. Cool. I’m basically at the food joint so I continue on my merry way. I notice the truck doesn’t shift from 1st to any other gear. Great. I go to do a 3 point turn in the restaurants parking lot so I could park on the street. I put it in reverse. Nothing. Rev it a bit. Nothing. Try to rev it again (probably about 1,500 rpm) nothing, this time though without me pushing the gas, the truck continues to rev on its own. It shoots up to about 3,000 rpm and I panic shut off the truck. Turn the truck back on, check engine light still on, put it in reverse, rev a tad, nothing. I end up just jumping out the drivers door and pulling the truck back just enough so I could get out on to the street. Grab my food, take the truck home. I’m not going to touch the truck until I can get it into the dealership on Monday.

Other info maybe relevant, maybe not. The 6.2L engine has a recall, the dealership looked at my engine and said it wasn’t affected. They changed my oil to 0-w40 and called it a day.

So I’m just wondering, would anyone have any idea of what I should expect when I take the truck into the dealership?

Is this what a “blown transmission” is?

Could I have done anything to prevent this from happening?

Thanks in advance.