Low engine power and backfire

So I've recently been having engine power problems with my 2000 Subaru Legacy. In November, a engine power issue prompted me to replace the knock sensor, and change one of my spark plugs, which was misfiring. The symptoms were loss of engine power when trying to accelerate, engine vibration and engine backfire. This meant the car could not rev past 3000 rpm on the highway and couldn't reach speeds past 70mph. In addition, the lack of power meant it would struggle even on shallow inclines and had trouble keeping the speed above 50mph on hills on the interstate. But changing the knock sensor, cleaning the throttle body and changing the faulty spark plug seemed to fix it. The car worked like a charm. That was early January.

A week or so after fixing the car, I filled my gas tank pretty high, and immediately started smelling gas inside the cabin. This continued for a couple of weeks, but eventually died out. However, faint gas smells can be sensed even now at times, especially initially when turning on the AC.

A few weeks ago, I started experiencing the reemergence of the low engine power issue. However this was different. I could turn the car on, and drive for a couple miles with no problem, but, after about 10 minutes at highway speed, the engine would begin to struggle. After stopping at say, a traffic light, the engine would struggle to re accelerate, backfire, and nearly stall. Acceleration would be slow, and the amount of power would be delayed and erratic.

Recently it has gotten even worse. Now, after only a couple minutes of driving, the car loses a significant amount of power, has trouble climbing hills, and and experiences a lot of backfiring. I changed all the spark plugs and wires, but the problem is only getting worse. Also, the fuel economy has gone way down.

What could be the issue?

I recorded a video of me driving the car, showing the symptoms. It's a little bit long, but here it is.
https://www.youtube.com/watch?v=w7M4YjmIW58

The engine is labored throughout the video, but here are some highlights.

At 3:10 you can hear the engine labored in acceleration, almost choking.
at 5:22 you can hear the engine fail to provide power even when the throttle is pushed nearly to the floor.
At 5:52 you can hear labored engine as well as backfiring.
At 6:59 you can hear labored engine as well as backfiring. Note at 7:13 that the engine coasts at 2000 rpm


Thanks so much for helping me diagnose my car