I do not own a GX550, I did see one at my closest Lexus dealership and was impressed with what I saw, unfortenately, did not test drive it because I could not.
A few comments for your questions:
Unless you specifically want a 2016+ for a reason, anything 2013+ is good because that is when the major improvments were made. Mileage is irrelevant PR0VIDED really good maintenance has been done to the truck. That could help you expand your search and more options for price.
You mentioned 3 row seats, which tells me you need space... the GX550 will be more cramped than the LC200. The 200's are just simply big and NICE for space.
Nobody knows reliability of the engine, but it seems it pulls right and hard. The 200 will last WAY pass 200,000 miles, WAY more, provided good maintenance, and the three or four things that could need repairs. Honestly, for low gas mileage, is a wash, if you want/need more, go for the LC250, and that would be my choice if I get one , will be the LC250, even with all the hybrid uncertainty, at least I am saving in gas.
Price again, open your search, and you will find options. Yes, they are still expensive for a reason, but be patient. You never said why moving from a 2016 T4R, they are incredible and the engine DOES have less issues than the LC200's, and for the looks, you buy new cars, so if you got it new, I am sure you have taken good care of it. If it is space you are looking for, I WILL NOT get the GX550. That T4R will last you 20+ years, specially that engine, it is a GREAT one ! If you have other reasons, go for it, but for durability on a vehicle to last you what you want, the T4R will be GREAT.
In short, based on what you posted, I will not get the GX550 nor the LC200. If you MUST get one, I will go for the LC200 because what you posted. NOT because I own one. AND if you MUST get a new one, I will get the LC250, at least you will save on gas. I want a LC250 but made the decision to not overpay, I am SO disapointed of dealers, that left me dry... on the contrary, Lexus is holding his class act and not over charing , and I almost went for the GX550, it is incredible and I like it, but I will not get it over the LC250. If there was not Lc250 with hybrid, then I will say GX550 If you must have new. But again, based on your comments, neither...I will keep the T4R !
Not sure I helped, but I tried. ! LOL. If you want more help, post away !!! This is seriously THE best BEST forum in the whole net, for real. !
Cheers,
G