@Valnatron Store content works, or should work, with any kind of (legit) Sims 3 install, so discs, Steam, Origin, and Windows or macOS. But anything available through the store needs to be downloaded separately from the store, not from Origin.
Normally, I'd say go ahead and contact EA customer support to get your accounts merged. However, there are a few reasons to hold off. First is that the account that disappears in the merge will likely not transfer its store content to the new account; this kind of content isn't included in merges. Second is that the wait time for EA customer support is extremely long right now, given the combination of reduced hours and all the people stuck at home with nothing to do.
The other issue is that account merges occasionally result in a game that won't open from Origin. The cause is that the base game code is blocked or doesn't properly transfer. This only happens to a minority of people, but it does happen, and then the only recourse is to have support fix it. So that would mean waiting in line again... and to make things worse, not all support reps will acknowledge that this is even possibly an issue. You might end up having to try a number of times over the course of several days.
If you do still want your accounts merged, EA support is currently available for live chat and email only, from 9 am to midnight UTC.
https://help.ea.com/en/contact-us/
In your position though, I'd go with a disc-only install if at all possible. If you'd like instructions on how to do this, let me know, but the short answer is install while offline, and don't accept the EA Download Manager's offer to manage the install process—it will try to patch itself to a full version of Origin, which will then try to patch your Sims 3 install to the Origin-controlled version.
