So reading everything in the comments sorry to say but it seems like you’re too good natured to say NO even to family. Hide your keys and tell them no. Tell them to buy a new battery with the max cranking AMPs available for the group size of battery needed. Tell them to unplug any cords they have added(dash cam, phone chargers, after market accessories, ect). As for your car you probably need a new battery to start with, make sure the terminal cables are installed snug but not over tightened. Then see if that resolves the issues. If not either pull your own alternator and take in for testing or go to your trusted mechanic and just have them slap a new one in. If you jumpstart any cars in the future make sure you drive your car for a bit after to built back the charge(the alternator does that). And tell them NO and it means the foot it down NO more using your car.
Take that truck and love it. The rust isn’t concerning. Colorado is a low rust state compared to others like Florida. Have a mechanic you trust check it over first though to make sure they can’t find anything hiding. If Carmax is very reluctant even if they drive it there and you’re willing to pay for the inspection, RUN. That means they’re hiding damages or issues. Even with cash try to handle them lower. Carmax over prices their cars already.