You have quite a few options there. if you like the HDS setup, the new HDI Elite-7 Ti is actually an older HDS more or less repackaged. (Gen 2, correct me if I'm wrong?) These can be had without card or transducer for $850, bargain, particularly as you have you already have a card and transducer(?) just plug and play. And you then have the ability to use either item in either role, ie, if one loses it's ability as fishfinder or plotter, you can keep going by using the other as a combo by just replugging transducer/changing nav card.
But it's all up to you, any of those things you are talking about will work, just add lots of money. Carbon seems to be very hi-specced, and expensive. Asking people whether they would go with one or the other is very subjective, you'll get 15 different answers from 15 different people, all down to personal tastes, preference , and wish to spend. 9 will have a bigger picture than 7, do you want a bigger picture? You already have a 7, is the picture big enough for your needs? At least if you go with two separate units, you'll avoid having to switch pages to get the page-sensitive menus you'll be dealing with all the time on a combo. Starting to drive me mad, I'll be adding an Elite-7 Ti soon to the Elite-7 CHIRP I currently run, using the older one as the plotter and the Ti as the sounder, plus it will give me the bottom lock the CHIRP doesn't.