GX470's are spectacular. Coil conversions are cheap/easy/common and solve the rear air suspension problems.
Come to the dark side- we have lattes! https://www.facebook.com/groups/LexusGXOR/ https://lexusgxor.com/
You seem to have been spectacularly lucky on one hand and unlucky on the other.
http://dashboard-light.com/vehicles/Land_Rover_LR3.html
http://dashboard-light.com/vehicles/Lexus_GX.html
For what it's worth, I've done this:
1st gen Taco ->
1st gen Tundra ->
1st gen Taco ->
LX450 ->
GX470 ->
GX470 #2
No regrets in moving to an SUV, and Lexus build quality is lovely.
It is likely capable of monitoring five tires. I haven't looked at a 5th gen, but in Techstream there are five slots for TPMS IDs for a GX470 (same as 4th gen).
If you're planning on taking it off road, consider getting the rear diff in the GX470 rebuilt with a solid pinion spacer. A high mileage 8" rear diff with the OEM crush spacer is likely to fail eventually if ran hard enough.
https://lexusgxor.com/general:purchasing
Model year 2005+ for VVT-i / more power. It runs fine on 87 (but the manual asks for premium).
There are a few people in GXOR with over 400k miles on their 470's.
Probably valve cover gaskets as mentioned above - sometimes tightening the bolts a bit can fix it (they may back out over time), but you may also need new gaskets.
That is pretty heavy. I felt my FCP's were too stiff before my rear bumper / tire carrier (though the front was perfect with hidden winch), so the sweet spot is probably around 5k lbs?