I checked CR's reliability ratings for the Gen 10 Corolla, and it's a sea of bright green dots. Not a single angry-looking red dot to be had. $800 for an O2 sensor is sure on the high end, it must have been the "upstream" sensor, which is one of the more expensive sensors on the market at $300 - but it's not a simple sensor like you find behind the cat, it's an A/F meter.
I know there are indie techs that will estimate a Subaru H/G job at $1500, but if you see that number, run away, unless they're a good friend of yours and you know you're getting a bro-deal. It's not technically difficult, but it's an involved job with a lot of steps that requires time. I thought about doing the ones on our Forester, as I've done a few on other cars, and ugh, what a job. Those damn heads are right up against the fenders, what a PITA. I used to curse Porsche for using the boxer when it was service time, but at least on the older cars they made it pretty simple to drop the engine. I remember my wife's reaction when I told her the car required the engine to come out for a major service. Wish I'd got it on film! (though the language was for sure NSFW)
With all that's required, you sure don't want a botched H/G job, so most people either use the dealer (expensive!) or a specialized Subaru indie, and they usually cost close to what a dealer charges.