We've decided to hire the CK Shuttle service that @Emily recommended - thanks Emily!
CK Shuttle, https://www.ckshuttle.cz/transports/transfer-from-vienna-to-prague/
CK Shuttle Direct door to door (not private) in van with 8 pax total:
€113 for 3 pax from Vienna to CK 2850-5400CZK
€113 for 3 pax from CK to Prague 2850-4950CZK
CK Shuttle with sightseeing stops (private car)
€256 for 3 pax Vienna to Melk to CK 6500CZK 11:00 - 16:00, :60 in Melk
€300 for 3 pax fm CK to Telc to Prague 7250CZK 11:00 - 16:45, 1:30 in Telc
Rental cars are SUPER expensive when not returning to the pickup location. For trips across county borders, the fees were €300-400 more than the already high rental fees. For a 3 day economy car rental, Vienna to Prague, it was going to be more than €800 total without insurances.