I checked Cura and the Silhouette Alta is not in the official list of printers but that is only an inconvenience. Cura offers a “Custom” setting which you configure. Apparently some have tried this successfully but they didn’t share the details.
https://community.ultimaker.com/topic/23530-cura-support-for-silhouette-alta-3d-printer/
By the way, I have a Chinese clone Delta printer with Marlin firmware – very similar to the Alta but full-sized. So it is set up as a custom printer. I have used a couple other slicers in the past but have only used Cura for a couple of years.
Cura will output an STL file which the Alta software can import. I usually put the STL on an SD Card and stick it in the printer. It might take some time to get the settings figured out but it looks doable.
Mark