Engine cutting out after basic maintenance

My Car: Subaru Crosstrek XV 2015

I did some basic maintenance today. I was experiencing no major problems before what was done today. Changed the oil and oil filter (5qts 0W-20 and Purolator Boss filter), cleaned corrosion off the battery terminals (baking soda and water, dried it, added dielectric grease and cloth pads), and saw that the coolant level was low in the radiator (the radiator proper, not the radiator reservoir). So I filled it up to the cap with Zerex Asian Blue coolant. The reservoir was empty and still is (It's my understanding that it's fine if the reservoir has no coolant in it because it's just for overflow for a pressurized system).

I started the car and let it run. There were no leaks under the car. So I took it around the block. The engine lost some power backing down the slope of my driveway in idle, but it didn't cut out. The car died on a downhill slope turning back onto my street after going around the block in idle. It started back up fine, and I limped into my garage.

I ran the car in neutral. It had inconsistent RPMs. Fluctuated between 1400 and 1900 at idle. Revved the throttle a little and the RPMs dipped down to around 1000. Revved it a little more and the RPMs shot up like it was overcompensating. Then letting it idle made the RPMs dip even lower to around 500.

I thought it might be a problem with radiator fluid leaking through the head gasket. Since the radiator was the only thing I could think was the problem (I had it replaced earlier this year after the old one cracked open). I checked the radiator fluid level and it was still up to the cap. Put the car on an incline and ran it with the radiator cap off. There was no change in fluid level, and there was no bubbling indicating that there was gas in the line. The exhaust was also normal; no white smoke.

Saw that the air intake hose was not fully clamped down for some reason. So I fixed that. I cleaned the mass airflow sensor just in case. No change in the RPM problem. Ran the car again and noticed a sound that might have been the fuel injector? It sounded like a high pitched squirting from inside the car. My girlfriend was at the engine bay and says it sounded like a clicking. Then we smelled fuel. Turned the car off and checked around. No leak we could see.

Anyone got an idea of how I messed this up? I don't know what would have caused this issue. Could disconnecting the battery cause fuel injection issues? Could it be a throttle body/intake valve/vacuum pump problem? Don't know why these problem would be exacerbated by what I did today.