For me, Ponant is the perfect mix of intinerary, price and comfort.
It is not for everyone - this was my third Ponant cruise, complaints about the food are common and based on the mis-understanding that the French eat souffle, foie gras and French onion soup every day. Simple, high quality fresh ingredients is the norm. Also expect the Captain, officers, receptionists, spa staff and some of the naturalists to be French by birth and speak English with French accents! It is a bi-lingual staff and crew. Some of the rudest Australians I've ever met have been on the Ponant ships, demanding that French not be spoken as they feel that they are missing out in some way. If you're that paranoid, there are many other 'English only cruise lines. Rant over.
The Kimberley expedition was Darwin to Darwin, which might be the first and last time Ponant offer that variation. The sea conditions and weather were spectacular - the calmest seas I've ever seen, clear blue skies and emerald green seas. Getting up to the Wandjina art caves at Raft Bay takes some physical fitness, as does the climb to the top of King George Falls, as does some ability to cope with the heat (35 Celsius on average).
Plenty of space for a solo traveller. Drawers and a sizeable wardrobe so that everything can be tidied away. The only thing I would like is for the ottoman at the desk to be replaced with a chair with a back.