What about the Gigabyte h110-d3a ?
A lot cheaper than the Asrock and it looks like it was made for mining aswell.
Don't see many miners using it though.
I took the Gigabyte Z270P-D3 when it was on sale, about 65$, tested with 6 GPU without any issues.
Maybe can handle 7 GPU using the m2 connector, didn't test.