New merging approach. I finally did it after approximately 10 failed attempts and some development stagnation.
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Personal note:

I got a new SSD and just moved my entire workflow from HDD to SSD. There's only one thing I want to say: IT'S FREAKING FAST! I literally had to wait for 10-15 minutes to perform a one-time merge on HDD, but now it's like FREAKING 90 SECONDS!?!? Having the new SSD also makes me daring enough to try a new merging approach.