Re: The best MODS for Sims 3?
@SuperNTG You'll need to explicitly set the other sim(s) within the active household as "unselectable," which you can do with NRaas MasterController plus its MC Cheats add-on. To do so, click on a sim and select:
NRaas > MasterController > Advanced > Selectability
Sims that are not selectable will be governed by StoryProgression. I've never actually tested this with only a pet as active, so I can't tell you exactly what you'll see, but I do know the overall concept works.
For the age range, it really depends on your preferences. I personally would suggest starting with either aging off or the lifespan on Epic, then decrease it if and when you decide you want the sims to age more rapidly. I mostly play on a normal lifespan, but I actually like the hectic results; you may have a very different response.
Keep in mind that once you've loaded a save, you can adjust the length of any individual age stage, for both humans and pets. Shortening it may cause temporary errors when sims that weren't due to age up yet are now overdue, but with StoryProgression and Overwatch installed, the mods will fix it on their own; there would be no need for you to do anything except wait a sim-day or two.
As far as other expansions go, I'm not sure that any specifically further your goals. It's more about what you want to see in-game. Do you want occult sims (Supernatural), or street performers and outdoor concerts (Showtime), or extra hangout-style community lots like a coffee or comic book shop or bowling alley (University Life)? Do you want something simple like more interactions available, especially for children or elders (Generations)? Do you want seasons?
Given your playstyle, I think it would be best to skip World Adventures because you'll get almost nothing out of it without an active sim—no travel and no access to the WA skills unless you deliberately buy and place the related items, all of which are found in buydebug but don't otherwise exist in homeworlds. The same is true for Into the Future. I love Ambitions, but it's not necessarily that much fun to watch inactive sims pursue "active" careers rather than controlling them, and all the fun of sculpting and inventing comes from having your own sim make things. Island Paradise is fun to think about but doesn't add a lot for you to watch passively either, and the world that comes with it is somewhat broken out of the box.
I suppose my cautious recommendation would be Showtime and University Life, in either order, with the major caveat that this depends in large part on your own preferences.