Brainslayer is (and has been) a complete nutjob for decades. To say DD-WRT is an "open source" project is stretching things to breaking point.
To get a device "supported" by DD-WRT you have to send Brainslayer a router. Nobody else can build the project because he intentionally obfuscates and breaks the build process as much as possible to "prevent people from selling DD-WRT" (as you're proposing doing here)
Once he receives it. He simply takes the vendor GPL firmware tarball and glues DD-WRT's UI onto it. There's no improved security for a DD-WRT device. Particularly the Broadcom ones back in the late 2000's/early 2010's when DD-WRT was more popular, Brainslayer never bothered to update from the 2.6.18 kernels the devices came with which left the Wi-Fi performance absolutely lacking.
OpenWRT is a completely different bet. But then you're fighting with GL.iNet who are literally a Taiwanese OEM selling OpenWRT-out-of-the-box devices