Solar is extremely reliable as long as you have an oversupply when all you want to to charge batteries. At sub 2c/kWh it’s highly variable but build in 50% extra and you’re only at 3c/kWh and now very consistent across a full week. Obviously, there’s varying tradeoffs so if the cheapest solution is to stick them on a boat and charge em in another country that’s what would end up happening. There’s also obviously tradeoffs between moving shipping containers vs moving electricity but again that’s an optimization problem not some inherent limitation.
Skysail isn’t viable at current fuel prices, the technology works just fine. Current sail designs are of limited value along current routes, but those routes are also optimized for fossil fuels.
Club Med 2 built on 1992 was 15,000 tons and a quite traditional design for aesthetics, again Skysail can significantly boost propulsion. Thousands of economically viable modern cargo ships are 50,000 ton panamax ships and plenty are well below that, those 250,000 ton monsters are an economic optimization between a handful of major ports not some inherent requirement.
The total crew size is largely independent of cargo size, but even relatively small ships are moving 100’s of TEU per worker. At that point it’s only a question of what’s the most efficient design economically not what’s the largest ship.