"I've been buying cheap no-name rotors and resin pads on Amazon."

"I haven't been using any oil or chain lubricant on the bike."

problems found.
the solutions should be obvious.

Youtube has multitudes of videos on how to maintain a bicycle... none of them recommend neglect.
Start by cleaning the entire bike... the cleaning can be an inspection, too.
Make a List of loose or worn parts that need extra attention.
use Windex and a rag, or quality paper towels.. clean the spokes too.