@Hervé you are absolutelly right. May be my memory misleaded me with Legacy Boot when setting up the DVMT on other hack long time ago.
Soo the problem with the grubshell was something messed up with the plist. I redo everything manually and finally was able to set the right parameters. 4K@30Hz is working without any problems now. Thank you so much @Hervé and @BillDH2k! I added some properties in DP to fix the finnicky HDMI hot/cold plug and now it's working well for both. Here's a screenshot of my DP with the properties I found to be the best for the moment.