How to make the seam of a Quartz counter top disappear as much as possible?
The only way to truly have a seam-free countertop of that shape is to cast it out of concrete. Since you've already chosen quartz, that doesn't seam like an option.

Embrace the seam, don't hide it. It's un-hideable, so anything short of perfection (which is impossible here) will be a disappointment and you will feel bitter about your multi-thousand-dollar expenditure because you didn't get what you wanted. Instead, change your expectations and design the countertop so that the seam is proudly visible and visually attractive. A diagonal seam at the corner could look nice.