Hi Ammonia, and welcome
One retry at cold start is normal and intentional for recent Maximus MBs.
Have you tried the SSD in other Intel SATA ports? There is a BIOS adjustment Tweaker's Paradise > SATA Drive Strength that may help the SSD be recognized. Adjust the SATA Drive Strength by steps of 1 and watch out for other SATA devices being missed. The adjustment may have to step either up or down. There is some chance the crashes happen as a result of misread SSD.
I see two other possibilities for these crashes, although others may see more -- power and heat.
A 500 watt PSU is barely at the recommended rating for a GTX970. If the GPU and/or CPU are overclocked, power demand could go higher and the PSU could drop out to protect itself. This possibility is less likely since the crash also happens with the internal Intel graphics. You could also check the mains supply for steady power when one of the crashes happens.
What CPU cooler? An overheated CPU will also stop to protect itself when it reaches 100C internally. Even when running stress tests, CPU temperature should peak no higher than the 80Cs. The 70C range in games is normal for peak temperatures.
Jeff