As you point out, to preserve design conditions (axle position, wheel camber, speedometer and odometer accuracy), you need to adjust sidewall size down to preserve the overall wheel diameter. Bigger rims mean lower sidewalls mean less ability to handle shock, so the suspension must be up to the task. Also, if you electronically adjustable suspension you may want to see if this needs to be adjusted. Also, to preserve ride you might want to see if other have changed springs and such.
We just bought a used 2018 Premium (17 inch wheels). We chose it over a 2016 Touring. Both great cars. We chose the 2018 primarily for the Toyota Safety Sense, but also specifically to avoid the 18 wheels in the touring. Read all the reviews, but most reports state that bigger rims may give you marginally better performance, and the will give you a stiffer ride, and the are mostly chosen for aesthetic reasons. They do look spiffier (IMHO) than the smaller diameter rims. In some comparisons, the dry pavement performance of larger rims is very slightly better (but in the wet, the 17 inch rims rule).
To preserve a lot of the car's design performance of the vehicle you must keep the same overall tire diameter. So a larger rim means a smaller sidewall. This means that the tire has less ability to absorb shock. Also, I suspect that the lower sidewall, with the same tire width, means that you have to have a stiffer tire casing (otherwise, the tread section would balloon out and you'd end up with a larger effective tire diameter - and larger sidewalls, effectively).
So you have the tire absorbing less shock. What makes up for this? Well, first you have a stiffer ride. I guess if you are steering through chicanes at 110 mph, that's maybe what you want. IMHO, the standard 17 inch rims give a ride in the Avalon that's plenty stiff, and they give good handling. But to soften the ride for 18 inch rims, the suspension has to absorb more energy. So bigger rims beat the suspension up more. I would suspect that Toyota uses a slightly different suspension for the larger rims. Or at least different tuning. Different springs? I'd check up on these things. A car is a system, and changing one element (rims and tires) to improve some aspects of performance may not jibe with the tuning (suspension, steering, etc).
Last, I think that low profile tires are more expensive to replace.
Car and Driver did a test of different size wheels in 2010. Pretty much no advantage either way as shown in their chart. See here
. 18 inch rims gave marginally better (5 ft in 133 ft total) braking distance.
Tyre Review did a test, too. Mixed results. 19 inch had better dry roadhandling. But 17 inch were better on wet pavement. See here
. They did say that on the vehicle tested, they preferred 18 inchers.