Just back from holiday and already planning the next one 😀

Do any of the European resorts keep the lifts open until the middle of September?

In Haute Savoie / Valais you have uplift through September at Chamonix, Verbier, St Luc, Zermatt, Saleve, Dorenaz and a few other even smaller places. They all have official bike trails, but none of them are worth a week long trip for that alone.
The non bike park stuff on the other hand, much better!
Don’t discount van uplift holidays for September, the weather can be a bit wintery in the big hills by mid September (and can be amazing too), generally more reliable further south.

The Pohorje Bike park near Maribor, Slovenia.

St Luc/Chandolin and Vercorin/Chalais has it’s lifts open mid-September. There’s easily a weeks worth of singletrack riding including nadgery content. Let me know if you need any route ideas.

Verbier, Chamonix – schedules online, lifts do close down over the month.

Metabief (I think)

Many swiss resorts served by mountain railway which runs all year

All of Switzerland.

Excellent! Cheers for all the help.