I have no problem with Kromer being the OL coach. He can still be the running game coordinator if he is OL coach. IMO Trestman brings more to the table as an OC. All those great OL coaches you named are OL coaches not offensive coordinators
As for OC hands down Trestman. Kromer OC experience comes under Trestman and he didn't call plays. If both are available why would you pick the guy who didn't call plays. Even so it is about more than play calling experience. Trestman has valuable experience as head coach, QB coach, offensive coordinator, and has even won championships at a different level. That experience is invaluable to a young coach and a young QB. If you are building the best possible staff Trestman would be the OC and Kromer would be OL coach. If the rams can surround Goff with three guys with extensive QB coaching and playcalling experience they should do it.