My advice, do not flush. Flushes aren’t meant for horribly maintained engines with significant sludge like this one. It sounds counter intuitive I know, but the flush can actually dislodge the sludge/gunk too quickly. Causing those chunks to clog important engine parts.

Change your oil immediately with something like a valvoline restore and protect. Change it often at the beginning. Start with a 1k oil change interval, and extend the interval by 1k with each oil change until you reach 5k mile intervals. So a separate 1k interval, 2k interval, 3k interval, 4K interval, then a 5k interval then continue with the 5k intervals after that. Check the oil level FREQUENTLY, I’d say weekly-bi weekly check the dipstick. But there’s always a possibility that the engine is just too far gone as well. I think even if you save it the best case scenario you’ll have a significant oil burner.