newbie question
is there any merit in keeping the bed hot to try and prevent or slow the cooling of the upper layers?..
Yes, particularly if you have an enclosure which can heat up slightly.
There’s a lot of research rabbit holes that can be gone down here depending on the material, different methods for keeping an even temperature to minimize gradients, etc.
I’ve read a couple of comments implying that a heated bed is not essential for pla.
If thats true then I assume the temperature range can be reduced (minimize gradients)
Im just trying to reduce warping of large prints (doughnut effect)
it’s not essential, that’s correct. I printed for a long time on a printer without a heated bed. However, a heated bed helps adhesion to the plate and reduces warping by reducing gradients, as you noted. It can also, sometimes, result in stronger layer adhesion.