Maybe caliper pistons sticking?

Or sliding pins seized.

New discs are cheap these days, if you can wait and take advantage of special deals you can get really good quality parts like Brembo/Apec for the same price as unheard of job lots from the usual auction sales sites.

If there's an appreciable wear ridge or a number of wear grooves in the disc then i will always renew the discs at the same time.

The general rule is you can put new pads on old but serviceable discs, but it's false economy to fit new discs and re-use pads that are worn to the shape of the old disc, though the OP at 500 miles hopefully the pads were not worn enough for this to be an issue.

When you fit new pads to older discs it's quite normal for only a part of the pad to be in contact until they wear to the shape of the disc, this is a time to brake gently and regularly and keep the heat down or you could cause a lot of issues by overheating the contact points of both the new pads and the discs...this is quite an uncomfortable time driving because it can feel like you've lost some 50% of your braking performance, because well you have lost it, if the wear lips are bad enough on the disc it might be the case that for the first few miles you are braking on some 10-25% of the pads friction area...fit new pads all round on old discs and it doesn't take a genius to see how poor the braking will be.

New pads on new discs, correctly serviced calipers, literally a handful of miles will normally see them fully bedded in.

The OP's issue i am sure is due to fairly long term neglect of the braking system, they did after all get down to metal to metal contact, so i dare say it's possibly years since the brakes were serviced properly (strip clean lube with the correct brake grease), if they ever have been. Left long enough without maintenance and it's entirely possible the calipers could be beyond saving too.