Unless you have something a heckofa lot better than chemical engines, "a couple annual flights" for crew is not possible. You have to wait for the best launch window.
Ah, sure, I forgot the launch windows, but I wasn't trying to be specific on the schedule. Seizing the opportunity to decrease costs and accomplish more at once opens possibilities. I haven't got a clue what schedules they could use, but I am pretty sure something of the sort can be done if you've got the right price tag and the resolve.
They could rotate the entire crew every two years, leaving the ground station unmanned for a while. That would be all sorts of amazing, imagine how creepy it must be to go into a completely empty space station.
Also, how much travel time could be cut by constantly working ion engines? I get that the pre-prototype of the thurster intended for the cancelled Jupiter icy moon mission
could get the Destiny laboratory several AU away for a year operating at maximum power (50 kW), with only 200 kg of fuel. But I've no idea if you could operate it that way.