In general I prefer a lighter and synthetic core string on a solid body EUB. Since the bridge does not move as much as on an acoustic double bass, a steel core string might feel rather stiff.
Go one step less tension (medium to light or light to downtuned solo) and try a synthetic core string. Innovation Polychromes might be good if they are still available somewhere. But don't use a low action with synthetic core strings or they might rattle. Should be comparable to the double bass. Obligato might work too, but I dislike the high stretching of those.
I'm not sure about the tuners. If they are smaller diameter than DB tuners and less wide, synthetic core strings might not fit.
Go one step less tension (medium to light or light to downtuned solo) and try a synthetic core string. Innovation Polychromes might be good if they are still available somewhere. But don't use a low action with synthetic core strings or they might rattle. Should be comparable to the double bass. Obligato might work too, but I dislike the high stretching of those.
I'm not sure about the tuners. If they are smaller diameter than DB tuners and less wide, synthetic core strings might not fit.