For me, I create the characters in CAS that I wish to play. In fact, I created the Cantrell family for the mini-series in my siggie below. Since the only daughter is my O/C (Original character, for those not familiar with fan fiction), her brothers never got much attention being in the background. Bit parts. Erik is her favorite brother and I made him an aspiring musician in the story. Now, it's his time to shine. I have to use a mod for it, but he came into his own in Sims3 with Showtime. I'm very invested in him, to the point when things go wonky, as in the file gets corrupted, or he dies unexpectedly, or a new mod totally breaks the save, I simply re-start them.
And I swear to goodness that as soon as I get close to where things left off after the save collapsed on me, another patch drops. Face/palm. He is still in high school in my current save, just found his girlfriend (yes, I place the same families back in each time I have to start anew). This time, I've had some great help from other players. There is a way for him to win Starlight Accolades for his music. A little mod that squelches the cool-down on licensing songs. And it has to be a song, no lyrics it seems. That limits him to the violin. But, that's okay, he's level 10 on all instruments. He is a Musical Genius, after all.
I agree that Legacy play can be a bore. I did finally complete Pinstar's Legacy Challenge for Sims4. I did it in Sims2 and it opened a whole new world for me. I learned things about that game I never knew. Plus, I had many firsts occur. A Sim got abducted by aliens, for instance and had a male child as a result. He became my heir and in Uni was kidnapped and brought into the Secret Society. Way cool stuff. It took a solid year for me to get through the Sims4 version, only because I wasn't playing every single day. But even though Erik was my Founder, he still couldn't sing. We didn't get that ability as a real skill until City Living. I got totally hooked on Erik. I think I'm addicted, in fact. LOL
Longer story short, find or create a Sim you 'fall in love' with, essentially so that everything that happens to them matters to you, too. That's how I stay in the game.