I think the ideal here would be what good trailer brake adapters do in cars that are not factory equipped with a trailer light connector: they have an adapter for each car model that plugs directly in to the stock wiring harness at the taillight. No soldering, no stripping wires, no piercing insulation. However, this complicates the product because you need an adapter for each bike manufacturer and possibly each bike model or even model year.
Another advantage of this is it simplifies installation for the bike owner. Just mount the controller and connect the plugs.
You have to run the business case (at the end of the day all the work you do has to support this).
What's the NRE/tooling and marginal cost to produce a harness variant, and how many units with that variant will you sell? Will people buy the device without the harness adapter?
OE connectors are hard to get, they're typically not some stock Molex or Deutsch dealy-o. I've worked on projects where we 3D printed whole connector blocks to try to do get mating to OE ECUs. It kinda sucks, and you need a whole lot of volume to support that kind of engineering.
Agree, it has to be practical. The better trailer light adapters do it but probably not for every obscure older car/truck model. And really the whole product idea is something that only applies to older low-end bikes. Newer/higher end bikes increasingly have this feature from the factory.
Another advantage of this is it simplifies installation for the bike owner. Just mount the controller and connect the plugs.