I've since completely rewritten the custom DSR resolution guide and tool. The process now looks like this:
1. Export your Nvidia DSR registry settings.
2.
Use this tool to generate the registry data for up to 10 custom resolutions.
3. Swap that into your exported registry file and save. Double click that file to import your registry settings.
4. Use DevManView to toggle your display adapters so the new settings kick in. This saves you from having to do a restart and only takes about 10 seconds.
5. Enjoy your 10 custom resolutions in any aspect ratio you want.
I can now swap in 10 brand new custom resolutions in about 30 seconds to a minute. The longest part is manually typing the resolutions into the DSR tool
Detailed guide here.
Lovely 3:1, 2:1, 3:4, and 21:9 resolutions.