Recommended requirements for Sims 4 and all packs?

@waterywatermelon  Your processor is more than sufficient for ultra settings in all three games, as is 8 GB RAM, although it would be best to keep the multitasking to a minimum while playing Sims 4.  You could also add more memory if you wanted: it's about the easiest upgrade there is.  I don't know what kind of hard drive(s) are in your computer, but given its age, you might have no solid state drive at all, or else one that's too small to hold your games.  In that case, adding one and installing Sims 3 on it would make for a significant improvement; Sims 4 would be less affected, and Sims 2 probably not at all.

You could also upgrade the graphics card, but your options would depend on the power supply and whether it has (or you're comfortable adding) one or two power cables that would run from the PSU to the card.  If you're not sure, just list the specs of the PSU: manufacturer, wattage, and rating.  The space inside the case also matters, specifically the length between the back of the case and the first obstruction, measured along the length of the graphics card or its slot.

If you're thinking of getting a new system, of course none of the above matters.

Sims 2 is the least demanding of these games by far, so it's no surprise you're able to play without much trouble.  Sims 3 is actually less forgiving on lower-end systems than Sims 4, although the requirements for ultra settings are essentially the same at this point.  As for why slowdowns happen in Sims 4, yes, there are plenty of other reasons, probably including some sim generation as in Sims 2, but I haven't really looked into it.  In Sims 3, the game seems to create new NPC sims as a save is loading, so you wouldn't necessarily see any lag, just a loading time that's a minute or two longer than expected.  Lag usually comes from the game engine trying to reroute stuck sims, or from similarly endless loops, but in many scenarios, if the issue didn't resolve, you'd llikely see a crash before long rather than extended lag.

As for why Sims 4 doesn't have a convenient way to uninstall individual packs, I believe it has more to do with Origin than anything else.  Most Origin games' DLC has a trivial impact compared to the base game, and therfore there wouldn't be much cause to uninstall that DLC.  So the feature was never built into Origin.  It's not currently in the EA Desktop App either, although that's still in beta.  Disc installs predate Origin, and the installers were often built for their specific game, so there was more room to add features that only that game might need.