More tire sidewall is always going to ride smoother. If you stay with the same tire model then performance should stay just about the same.
However in this case I suspect you're also feeling the difference between two different tire models. What tires do you have on your 19" wheels? The 18" aero wheels usually come with Michelin MXM4 which are a smooth-riding, low-grip, low-performance tire focused on efficiency and tread life. They have soft sidewalls and will definitely smooth out the ride vs most performance-minded tires, at the expense of handling, grip, and road feel.
Soft sidewall tires like snow tires or eco-minded MXM4 will accentuate the extra sidewall, but good performance street tires will have stiff sidewalls and still feel very responsive in 245/45R18 or 235/45R18 (the recommended tire sizes for a Model 3 on stock-sized 18x8.5" wheels). With stiff sidewalls they won't ride quite as smooth as something like the MXM4, but any tire will still be smoother with extra sidewall.
If you're after a smoother ride then switching to 18x8.5" wheels makes sense, you will get a smoother ride and cheaper tires with no downside.
If you have somewhat stiff performance-y tires on right now you can also switch to a smoother riding tire, but that will probably come with a performance downgrade, which you may or may not feel or care about in the driving you do.
Reference: I've used smaller diameter wheels on 3 cars now, including my 2021 M3P. In two cases I switched to smaller wheels permanently, the third was as winter wheels with snow tires. In two cases I dropped 2" of wheel diameter, the third I only dropped 1". I've also changed tires many times without changing wheels of course, between same-category and different-category tires.