Hello Michael,
I found I was able to eat all the things you mention once my j-pouch settled in (around the six-month mark), but my j-pouch is not bothered by sugar, so you might want to go slowly with the dried fruit. Also some people find the fruit skins a bit indigestible even on dried fruit. Maybe sprinkle a bit chopped up small on something you know you can eat.
A note of caution with mushrooms: make sure to chew them properly, or have them cut up smallish or sliced finely. It is easy to swallow a larger chunk without chewing, and occasionally people get into trouble with this. (I never had anything as serious as a blockage even though I gulped down a few mushies accidentally but it can happen). Same with steak, I believe – small pieces and make sure to chew.
Re weight gain, I used to eat a baked potato with a rich topping like grated cheese, butter, or sour cream. Maybe worth consulting your doctor or a dietitian about your lack of appetite.
I don't know whether this will apply, but sometimes you can get advice from staff at the hospital where you had your surgery, e.g. stoma-care/pouch-care nurse, dietitian.
Sorry to hear you can't get outside. At least perhaps you can open some windows and enjoy some unpolluted air?
Best wishes from Down Under,
Sarah