Every couple is different. How are you two (and the dogs) on long car trips? Most boat cruising, especially on smaller boats, isn't endless hours on the water - you find great places to anchor or dock and go exploring. When the weather turns bad, that is when you find out how you do in confined places.
I used to joke with people who asked about how we were able to cruise for months at a time on a smaller boat, and would say, "Lock yourselves in the bathroom for the weekend. Bring in a cushion to sleep on, and have something to cook with... that will get you started. Of course, when the weather turns bad and you're on a boat, there isn't any way to simulate that with your stationary bathroom." 😉
Some people couldn't imagine traveling together on a 40' boat.
My wife and I have been together since we were in high school. We motorcycle camped when we were younger. Traveled extensively in a couple different campervans. Moved up to bigger motorhomes, cruised on smaller boats (from 17 1/2 feet to 27 feet). With a properly configured 21 (including a camperback to add living space to the cockpit), I would think it is doable... for us. You'd have to weigh your situation to know if it would work for you.
Good luck with the "journey" and the decisions,
Jim B.