Yes paved 2 lane roads. ie tarmac not dirt.
Potosi to Sucre, Uyuni and La Paz are all good roads.
I'm not sure on the schedule from La Paz to Potosi, but the overnight La Paz to Sucre bus stopped there early in the morning. That overnight bus cost about 135B and was full cama (lie flat beds)
Which bus company did you go with? Was it Todo Turismo? I am planning to vist the salt flats on March 12th or 13th of this year.
Todo Turismo operate the tourist bus direct from La Paz to Uyuni. Its not a full cama bus service.
The La Paz - Potosi buses usually leave in the evening (7.30-10pm approx), its about a 9 hr trip. A few companies do it. They arrive at the newish bus terminal on the outskirts of Potosi - get a taxi to the Terminal Provincial (the old one) which is now where services in Potosi department, including to Uyuni and Tupiza, depart from.
The La Paz - Uyuni buses (I assume Todo Turismo as well) are currently going via Potosi anyway due to rainy season and the bad state of the direct road