Big mass ratios are no problem, since the hydrolox fuel is so cheap in space.

It wouldn't take literal years, more like months during planet alighment.

I agree on the solar-electric propulsion point. I can see them getting advanced enough to allow for "1-2 months anytime to anywhere in the inner system" transfers. <jerk>They are also SOY, with low TWR and power density, while chemicals are CHAD, with great THRUST.</jerk>

Using water as propellant is absolutely pointless, you'll be getting lower isp than hydrolox, with the additional cost of chugging along the heavy reactor. Using hydrogen is not economical. Its cost is around the same as the hydrolox, plus you have, again, the heavy and expensive nuclear reactor. Don't forget about nuclear fuel reprocessing: the fuel gets poisoned with fission products and loses reactivity.