My tandem is a nightmare on gravel – mostly because putting a couple hundred pounds on one tire means they sink deep into sand. Sand is a nightmare. For gravel, I would want big fat 26” tires (55mm or more). I have a thudbuster (ST) for the stoker. I love my ergon (on my gravel bike), but think I would want the extra cush and travel of a thudbuster on my bike.
Still, I should try to put as large a tire on the front and try it some more. The tires tend to need much more pressure than road tires because of the weight they carry
(60mph? I’ve overheated rim and drum brakes doing 40 going down hills in Europe – that is always an interesting experience – fortunately the tubes take a couple of seconds to lose all their air).
(I don’t know how old my Trek is, but it’s a 21 speed, lol).