This is great, you are doing exactly what we had hoped people would do. Use the DragonBoard as a development system for your product, and then move over to a custom design for your final product. The teams at Arrow can help you with supplying an appropriate memory chip. SanDisk doesn't post their datasheets on the web, you need to talk to SanDisk to get a copy of the datasheet.
The DDR settings are stored in a tiny partition called the CDT, which can be changed without rebuilding the proprietary binaries. Again Arrow can help you with this.
I will ask the teams for a suggestion for an alternate memory. One quick question, are you interested in a memory module ( called a MCP, or Multi-Chip Package) that contains both DRAM and eMMC, or would you prefer separate DRAM and eMMC chips?
Just a quick reminder, when you are designing your own board you should carefully read the design guidelines: https://developer.qualcomm.com/hardware/snapdragon-410/tools There are also third parties such as www.valydate.com that you can contract to do design verification.
Full disclosure: I am an employee of Qualcomm Canada, any opinions I have expressed in this or any other post may not reflect the opinions of my employer.