I just replaced the engine and a code P0172 turned on?

Just to catch up, I changed my previous engine because 16 years ago it was turbo'd (found out from a YouTube video) and it was abandoned on the road, towed truck got it and claimed it, sold it to us. Had a lot of issues but my dad and I just addressed it. But it was given to us without a kit installed and gauges. Probably were like, the 5th owner I'm not sure lol. Because it was turbo'd, it caused a blown head gasket, and after a 1000th time of overheating and multiple cheap fixes, my dad gave in, and we replaced the engine.

My dad was driving on the highway with cool temperatures on the new engine and didn't overheat once. But then it started struggling, as if the serpentine belt snapped and started struggling (it didn't). He pulled over and started it again and it backfired before starting normally. Checked codes with the OBDII scanner, and he said, "something with the catalytic converter" and a p0172. This was about his 7 or 8th time driving it after replacing the engine last month.

Also to mention that when we started the car for the first time it started at a very high idle, like around 1,500 then got up to 1,800 RPMs. After driving it a few times it slowly went down and started at 1,000 RPMs and stays at 1,000 RPMs unlike my 06 xb where it starts at 1,200 RPMs and climbs down slowly. Its code p0505 for the IAC valve, I replaced it. But still have slight normal idle issues, it would only start and stay at 1,000 RPMs. I replaced a lot of vacuum hoses with blue silicon hoses and still the issue persists. I used a cold air intake for a little before I switched back to OEM for testing.