Taxi is available the whole night (or at least until the festival arena close). The festival cooperates with multiple taxi companies and there are even taxi drivers from other cities just for the festival.
Public transport in Czechia is super reliable. There can be some delays sometimes but it gets rarely cancelled. And the delays are more common for long-distance trains/buses than for regional lines. In this case, buses are much better option as the train station in Opo膷no is quite far from the town centre. Tickets are sold by the bus driver. You would have to get a taxi back to Opo膷no though as there are no buses or trains till the morning.
The weather is usually nice. 2022 was sunny but a little bit too hot. 2023/24 - warm during the day but cold later in the night, light showers (just for a few minutes, nothing horrible). Wellies are not needed...
Not always one for Pyramid headliners depending on who was on over the eight times I've been but:
Stevie Wonder 2010
Coldplay 2011
The Cure 2019
Stormsie 2019