It's not likely that you'll be able to plan this until you get there. Bus companies are constantly changing, as are their routes. A lot of companies don't even let you book until a few days before, and you need to go to their offices to do so.
This is a long trip to do all in one shot. Personally, I would go from Dar to Nairobi, and then to Kampala. Unless you're wanting to visit Mombassa specifically.
Stay the night in Nairobi, there are hostels near the streets where the busses stop. If you get off at night, jump in a cab immediately, the bus area is super sketchy, and tourists get robbed there in broad daylight. You can bargain for your fair, start at 50% of whatever they say they'll charge. You can catch a bus the next day.
Alternatively, you could spend the day in nairobi and then catch a night bus. The night bus takes about 12 hours to Kampala, and it's pretty comfortable as far as busses go. The seats recline. I have never been on a bus in Kenya or Uganda with bathrooms. I would suggest taking water because it's dry, but drinking a minimal amount because sometimes busses don't stop at all. Though this one would have to because it crosses the border (and there is a washroom there).
Also, if you're in Nairobi on the weekend, there is a nice outdoor craft market and it's easy to find.