@DystopianViolet Every time you create a new household, it's saved to your library. If you want to re-save a household you've been playing with for a while, you can do this from CAS by clicking the "Save Household" folder icon. You won't keep personal inventory or non-household relationships, but I sometimes will re-save a household after kids are born or age up or a townie joins them.
When creating a new household, if you want to use a Sim you've previously created, go to "Open the Gallery" - the next icon over, which looks like a lightbulb.
When it opens, switch to the "My Library" tab
Choose the household you want to reuse and click "Place Household" in the bottom right corner.
You'll then have the option to merge (keep the existing Sim[s]) or replace (get rid of any Sims that were in the editor) to put a copy of the Sim you created previously into the new household.
Then you will have new copies of the previously created Sims, with editable traits
@DystopianViolet You have a couple options if you want to get rid of one Sim in a household.
1. If you're willing to use cheats, hit CTRL + SHIFT + C and type testingcheats true in the cheat bar. Then hold shift and click on the Sim you want to get rid of, and choose "Remove from family" (or "Kill" if you're feeling mischivious, though be warned the rest of the household will mourn).
2. If you don't want to use cheats, or have trouble like you did with fulledit in CAS, you can still have the Sim move out.
Open their phone, and go to travel:
Then choose "Split from household and move":
You can either give them enough of the household funds to purchase an empty lot somewhere, or move them out with nothing and just merge them into another household. You can then delete that household.
3. Of course, you can also easily split a household from Manage Worlds, rather than while playing the household. Find the household you want to edit, either in manage households or in the world. If in the world, choose "Manage Household"
When the household editor opens, choose "transfer Sims between households":
Create a new household and transfer out the Sim you want to get rid of:
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No matter how you split the household, the last step is the same - open manage households and delete them:
