You play like me, Franc! I play legacy on long lifespan, too. The only mod I use is Risky Woohoo. So I don't have anything populating the world for me -just the rare unexpected pregnancy in the household I'm playing.
I pick a "main" character for the next line of succession and focus on them. I'm doing something special with genetics in my legacy so the main has to come from a specific bloodline. Basically, my last main's bloodline (though my rules are a little tighter than that); so it's usually an option between only two children. I don't usually have a ton of kids so it's seldom a hard choice. I just select the one I find myself gravitating towards playing the most.
For the extra children and cousins and cousins' cousins. I move them out into their own lot if it's not already separate from my main household. Even without MCC Commander, your sims will often make relationships or gain friendship levels on their own. Especially if they're in a club.
I play my main household and around the time I'm expecting my next generation or planning on having kids soon, I go and pop into all the relative's households. Then, I marry them to the person they have the highest relationship status with. Sometimes this is just a little green stub, other times I've had them already have max relationship and romance status with someone, but they just weren't dating. Either way, I nudge them along into a romantic relationship with that person. I then play out a little wedding scene for them and have them try for a baby. Then, I exit the household. The wives go through the pregnancies themselves and the game auto-names the baby and generates traits for them as they grow up. Effectively populating the next generation for me.
I like throwing weddings, so I feel it's very little effort on my part -and I always invite my main to the wedding! So I still get pictures of my main focus. In fact, it's funny to see how they act when I'm not controlling them. And relative's romances themselves can often be as brief as a single day. So, I don't feel like I'm wasting too much time doing this at the moment. Plus, it lets me skip a lot of the "baby in a basket for two weeks" that I'm not always interested in playing in my main household.
Sometimes you'll pop in on that ONE GUY who is such a shut in that in his entire life he never made a single friend and knows like two people; both of them children. In those cases, I spend a little more time on them and go hunt down a random passerby from the street. But, I imagine, once there starts getting too many cousins for me to handle and I run into the shut-in. I'll just let that branch of the family tree die out so I can reduce the numbers once again into a manageable amount.
That's how I play it, at least :3
