<p>I have an UP! mini and I am using the UP Studio and ABS 1.75 mm.</p>
<p>My prints are always stuck to the rafts and are impossible to remove without destroying the print.</p>
<p>I've had a look at the settings on the Up Studio but I don't know which ones to change to improve the situation.</p>

FreeCAD can import JPG (and IIRC TIFF and PNG as well) image files and produce a lithophane type height-map based on the brightness of each pixel.  I'd be rather surprised if other 3D CAD software aimed at the 3D printing user base couldn't do the same.  Most of the common free-to-use 3D CAD packages have Linux versions; FreeCAD certainly does (I use it on Kubuntu 20.04, and it should work on any recent version of any flavor of Debian-based Linux, if your hardware meets it requirements).