There are some routers with OEM fork of OpenWRT that allow you to flash stock openwrt without any obstacles and by easily dropping the image blob in the web UI - just easy as upgrading the firmware from stock openwrt.

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

There is something that a lot of people looking at businesses like this fail to realize. Something like this probably doesn't scale. You can get reasonably Wealthy opening a small specialized shop just yourself selling to just a small niche of customers. However, this business doesn't scale: You will never be worth ten million dollars opening a business like this. If you are fine with being a one or maybe two people working in a shop and earning a good income but working 40 hours a week supporting your customers, yeah, it's worth a shot to open the business. You will never be as large as Google and so you'll never be worth billions of dollars but might be good business.

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.