What I have in mind (if my dayjob fires me): open a shop for wealthy nerds, and sell all kind of pre-flashed devices, known to work well with open source firmware: power plugs, routers, thermostats,... The clientele would be very niche, but I think it might be enough for a small specialized shop.
Granted, not exactly what you wished for, but pretty close.
The time to do this is <5 minutes. One such router I personally know of is GL.iNet GL-MT6000: https://openwrt.org/toh/gl.inet/gl-mt6000
The OpenWRT version is relatively old, and unfortunately the latest firmware requires reconfiguring family permissions, but I’ve otherwise been happy with the device. I use it for all kind of strange things (ad-hoc extending my home network for getting my solar inverter back online, tethering to my phone, etc.), but primarily use it to make it easy to get my family’s devices online while traveling (6 iPads, 3 phones, and a laptop).
Unfortunately I had to use might there and not Will. I do not know for sure if this is a good business. I would like to say I would recognize such a business if it was near me, but I don't know. I can do this myself. I don't know if you can make enough value that it's worth me paying. If it takes you three hours to flash a router, I can do that myself. And even if it takes me six hours, Three hours of your time at a reasonable wage is more than I could afford.