If after a Marathon, you’re experiencing recurrent symptoms of injury, then it’s probable that there’s underlying pathomechanics producing tissue stress or more severe injury. It is important to get a medical assessment for any persistent problems and then consider maintenance from astutely selected and structured footgear to keep training.
It’s difficult to make recommendations about post-Marathon footgear because it’s highly individuated. Generally speaking, lightwear sandal oriented footgear should have structured insoles and reinforced outsoles with stability designed into the sandal. Open-back is fine as long as the sandal stays with you without excess demand on the toes or forefoot. Your footwear should protect you from excessive shock or pronation while you recover.