I only play my master save file and I keep a backup. I've lost it several times and at first I'm always distraught but then I actually enjoy starting it again. I have the same Sims as before but as some were base game, as I've caught up with all the packs it's given me a chance to recreate them with the full game. They have the same appearance but are better dressed and I might change their look a bit. Their homes will be better too. Their core personality will be the same and they'll eventually have the same kids they had before too. New content will really add to their story, so for example my Spellcaster Aurora was just a nature loving Sim. But after I lost my save and had to remake her, I'd added Realm of Magic and Cottage Living so she was remade as a Spellcaster with wild birds, rabbits and foxes in her garden. Next time I had to remake her I had Horse Ranch and Werewolves. So now she has a Unicorn and is a friend of the Werewolves. My base game writer, Alla became a horror writer and now lives in Ravenwood. All of my OG Sims have been changed by the new packs I've added. Losing my master save turned out to be not so bad after all.
I've been working on this save for over 2 years now and my current project has been giving some Townies a life. I'm going through the core Townies that are friends with my Sims and giving them careers and homes. I had loads of empty apartments in San Myshuno so I've been decorating some and moving Townies in. I also linked it with Pride month with a few rainbow themed builds and Sims. That's been a really fun project. I go through the oddly dressed Townies and edit them already because those random outfits are not so great! I've also edited quite a few of the base game Sims homes too, like Nancy's and the Goths.
I also go through public lots and do a lot of work. My gyms are also spas and my parks are campsites and often have outdoor cinemas too. I've updated vacation rentals in the Jungle and Granite Falls too. I need to redo the bars and lounges though as I haven't done enough of them. I've also added shopping malls and retail stores in a few worlds too.
I'm hoping I don't lose this save while I'm just getting it how I want it 🤞.