Canada also seems to be pushing for smaller <100MW modular reactors. Would those change the equation?
The temperature is determined by practical considerations, particularly the coolant that is in use. The LWR is limited by the use of water as a coolant, but it doesn't attain temperatures as high as a coal burning power plant does because of materials considerations. The reactor has to hold up for a long time under a high radiation field and the results of a failing reactor vessel would total the reactor and lead to an expensive clean-up. (But wouldn't be an environmental catastrophe so long as water could be kept circulating in the confinement)
To raise the operating temperature we would need to switch to a different coolant, and that is easier said than done because that coolant has to be compatible with all the materials in use, not absorb too much radiation, not be damaged by the radiation, not be made radioactive than the radiation, etc. The physical form of the fuel is affected by this too.
For instance, LWR fuel can be removed from the reactor and stored under water for 15-20 years and then stored in dry casks for an indefinite time. We know we could bury it underground and it would stay there for thousands of years. We know how to dissolve it in nitric acid, extract the uranium and plutonium and use it to make new fuel.
The waste management story is much sketchier with alternative fuel forms.
People have built reactors with different coolants, a good selection can be found here:
https://en.wikipedia.org/wiki/Generation_IV_reactor
As for the "small modular reactor" that is going from the frying pan to the fire. The first LWRs that we built were small and it was clear that the economic case for them was bad and it was thought that it could be improved by "going big" but that didn't happen.