Hello Denlaw55
From my perspective, the Swiss Travel Pass Flex is rarely a suitable offer. Often, a regular ticket would be totally sufficent, even in your case.
The use of a Swiss Travel Pass Flex makes sense if the regular ticket costs more than CHF 87, what is the equivalent of a STP Flex day.
Basel to Lucerne, however, the the regular ticket in 1st class costs CHF 62, thus you are paying CHF 25 on top when using the Swiss Travel Pass Flex for that journey. In Lucerne, buses are free of charge within zone 10 of the local public transit network if you hold a hotel reservation or the visitor card that you can download. There is no need for the STP Flex.
Often, the Swiss Half Fare Card (CHF 120) plus individual tickets is the most effective and inexpensive solution in particular if you do not need the museum pass.
Kind regards,
Roland