Step one would have been not to have huge gaps in the corners tbh...

The job starts with hanging the drywall. The cheat is to use Great Stuff to fill gaps, use a filet knife to cut it flush and move straight to tape and compound...

But that's in the past.

You need to sand the highspots until your either flat or to the paper. Then start from there with tape and compound.

YouTube has lots of videos that make it look easy. It won't be, but with a few extra hours of sanding you'll still be successful.

Also, forget hot mud. It's hard to even know how much strength hot mud provides, it's more about working speed which isn't your concern. Premix is way more forgiving