Technically, it is possible to have only a spouse who will max painting, sculpting and photography in those 3 weeks while your founder is YA. Trick is to find a spouse quickly enough. I managed to do that 4 times, 3 times it was Zelda and once with Jamie Jolina, none was artistic and had no related skills. (Now I moved in Agness too so Jamie can max her career and have her LTW). But you're right, with that strategy, spouses don't do much but skill up and eat/bladder/sleep. You can reach level 7 painting in 3-4 days, and photography in less than 2 days, but sculpting is the hard and very long till you get 25 ice sculptures. It is still doable, so when founder reach adult stage, you get time for everything else (children, relationships etc).
Also, immortal museum doesn't have to be separate part of home, but it is easier to keep a track of everything, it can be even basement or simple wall at the back or the property, but once you lined all portraits and everything else (I also named them once I chose museum piece), you can easily keep a track and be extra careful when redecorating museum part.