Welcome @Janosch
Are you bending 14mm or 16mm for seat stays? The discussion about this thread is for racks, which are much easier to bend. Tubes that are .5in (12.7mm) and larger diameters will need more complicated bending setups to avoid the tubes rippling and deforming see this thread for some examples.
30mm center line radius (CLR) is very difficult to achieve. Typically people will only bend down to 3in (76mm) CLR
The best tube bender for stays is the cobra bender: TOOB Bender | Cobra Framebuilding
If you are located in USA, Andrew at @BikeFabSupply may be willing to bend SS’s to your spec: BENT SEAT TUBES — BICYCLE FABRICATION SUPPLY