Put synoboot.vmdk on the *second* SATA controller (so it is SATA1:0) and the storage disks on the first (so they are SATA0:1, etc).
Go into the BIOS and set SATA1:0 as the top boot device.
This worked for me on ESXi6.5 with "Jun's Loader v1.03b DS3615xs" and DSM_DS3615xs_24922.