Well, some of it for sure is just looks. The black Tundra wheels I have I think will look much better than the 21s. I also think a larger diameter tire more filling the wheelwell will look better.I did 275/70/18 LT Michelin Defenders and loved them. 285/65/18 is cool too. 295/70/18 is available too but will require some work to get it to fit.
21 inch to 18 inch will make a big difference but that's like for like. What are you trying to achieve with the new tire?
A bonus will be gaining the very modest height the new tire will give me. I might do the AHC lift at some point too.
I am inexperienced with overlanding and boondocking, but have an interest in it with my wife. Nothing extreme at first certainly. So while traditionally I've been 99% pavement, I want something that can handle a little more on. Living in New England, it also needs to be good in snow.
I had defenders on my Sequoia, and thought they were great. Though they were P rated 20s.
So I guess to sum, I want it to look better, not degrade the ride too much, but also be more capable for the unknown.