I just managed to successfully flash openWRT on my EA7500v2.
I flashed the OEM firmware "FW_EA7500v2_2.0.8.194281_prod.img" 4 times. As of this posting, its the one linked in the installation guide.
Then I flashed openWRT "openwrt-21.02.3-ramips-mt7621-linksys_ea7500-v2-squashfs-factory.bin".
The WAN port was not connected at any time and I did not need another device for DHCP assignment.
After successfully installing openWRT, I tried rebooting 3 times and never got booted back into the OEM firmware.
I had previously followed the installation guide and other comments about following a particular sequence but it did not work and I kept being rebooted back into the OEM firmware. Hope this helps others looking for a solution. Best of luck!