Yes, but sodium/potassium have their own share of problems like ignition/explosion in contact with air or water. Gas coolant can easily leak and doesn't have much heat capacity, lead requires high temperature to melt but it's otherwise non-corrosive, lead-bismuth eutectic needs lower temperature but corrosive. Molten salts are also corrosive, so reactors using them would most likely required to be built from special steel alloys. Using low pressure coolant with natural circulation is indeed a very attractive option.

Thorium is attractive because it's widespread and lower cost and supposedly doesn't have as many proliferation issues as LEU, since its fuel cycle also creates U232 along with fissile U233. U232 is highly gamma radioactive and thus quite difficult to separate from U233. OTOTH U232 would keep being radioactive for at least 700 years (10 half lives), so we woldn't want any U232 contamination occuring.