Gamecube Lasers are as known very picky.
There is NO GENERAL VALUE that you can use on all gamecubes.It depends how old/"burned-out" the laser is,maybe micro scratches.....you see not easy.I can tell you from my personal experience,i had many Gamecubes (purple/black/white) and NONE of them had the same setting.I go there,tear the GC apart and make it as i described above.And all of them worked fine.I´ll never use a multimeter.
only careful little steps by turning the screw with instant test.I´ll also make it with XBOX Classic and Dreamcast drives.And as you discribed,your GC Laser is fine while reading Games and normal DVD-Rs.A dying laser don´t do this.And thats what me thinking of your non compatible miniDVD-Rs.
But this is exactly what makes fun about modding/hacking consoles.
The hint to try another burner was great,please check it if possible.