I have extended on 10.12.4 on a 530 using HDMI and DVI outputs. You're not the first person I've seen on the forums stuck in mirroring and I have no knowledge to help you specifically, but I do want to point out that even on 10.12.4 I think you need the 4 port patch (the newer version: find 0103030300002002 replace 0103040300002002), and connector type patches to properly set the port type from the default DP (0004 0000) to the right type (0008 0000 for HDMI or 0002 0000 for DVI IIRC). You'll also want to make sure you understand the source of your config.plist if it's not the original clover version, and what changes you or others made to it because it's possible it's not 100% right for your board.
If you figure it out report back.