@Lori57 Wow, you really are not messing around. That's the kind of computer I'd build if I were doing so now except for the graphics card, but a 3060 will handle everything Sims 4 can ever throw at it without breaking a sweat. And when you're ready to add a faster GPU several years from now, the rest of the computer will be ready for it, no need to upgrade any other components.
I will say that if you're planning to play Sims 3 as well, you'll have some trouble with a 12th-gen Intel processor. The game doesn't start correctly on these CPUs, and while there's a workaround (limiting the game to one core while it loads), that can get a bit tedious. In that case, you might consider an AMD CPU instead. But if you don't play Sims 3, it's irrelevant.
As far as loosening the purse strings, I have no idea what you're up against, but what works for me (internally, when I'm arguing with myself) is calculating the price per hour of entertainment. Considering how many players have put literally thousands of hours into Sims 4, and this computer should be around for the entirety of Sims 5 (if it ever shows up), the price of a custom-built PC starts to feel pretty reasonable. I wish you good luck in your effort to persuade.