Link below has most everything you need. ( Btw, 17inch rims with matching tire sizes should work too. I see several people here with 17inch snow tires. However, it looks like you are looking for summer replacements.)
www.wheel-size.com
Offset is +40.
Check the the door jamb sticker for weight. Ours lists weight for rear axle at 3405LBs. I suppose that half of it would be per rim. I forgot the front one. I believe weights are mentioned in a couple of threads here.
I have noticed several people go with 18x8 rims. I have wondered how it affects the tires, but it looks like it is not an issue. I suppose it would just be tiny bit less tall with the tire starched half an inch more.

Toyota Sienna 2021 - Wheel & Tire Sizes, PCD, Offset and Rims specs
Toyota Sienna 2021 - Find out the correct alloy wheel fitment, PCD, offset and such specs as bolt pattern, thread size(THD), center bore(CB) for Toyota Sienna 2021
Offset is +40.
Check the the door jamb sticker for weight. Ours lists weight for rear axle at 3405LBs. I suppose that half of it would be per rim. I forgot the front one. I believe weights are mentioned in a couple of threads here.
I have noticed several people go with 18x8 rims. I have wondered how it affects the tires, but it looks like it is not an issue. I suppose it would just be tiny bit less tall with the tire starched half an inch more.