I'm back with a well-scratched head.
You have a well-set up MBP. I did the SSD switch about 18 months ago on mine and it was like getting a new computer. With 16 GB RAM your VRAM-from-RAM is very good.
Is Sims 4 a native install (like from the App Store) or are you running it through Steam?
I've looked up sysreqs and compared you comment:
...and I am still seeing lag in a particular section of my game.
to my sim experiences and that rang a bell. You may have something that can't be improved upon at your end because it could be the programming. Read on.
I have a naval warfare sim that, at certain stages, produces a transparent smoke effect. My old iMac with a 2.93ghz i7 quad-core processor and 1GB VRAM of hardware acceleration stuttered a little every time the smoke effect appeared, even though that computer exceeded all the sysreqs by a mile.
Now I have a 5K iMac with a 4.2ghz quad-core i7 and 4GB of hardware-based VRAM and guess what? The same spot still stutters to the same degree on a computer that benchmarks at 2.4X faster than the old one! The rest of the game runs fine. That suggests to me that the programing of the smoke is different.
I noticed in the Sims 4 Support site that there is something called "laptop mode." It is mentioned at the bottom of this EA support page:
https://help.ea.com/en-us/help/the-sims/the-sims-4/the-sims-4-system-requirements/#mac
It may be useful to try that, not so much as a cure but as a test.
Our MBPs don't have separate graphics hardware like my iMacs. It is shared with the main processor--that will always be a handicap. As far as I know, the shared VRAM at 1.5GB is as much as that processor can handle even with your extra RAM.
I'm not washing my hands of this yet. I have a few more things to check.
Allan