Thank you guys so much for your enlighted insight !
I did stumble upon JJBP conversion kit and they do have a specific model for the Backroad indeed, which is awesome ! Forkmods from Autralia is a reseller and provides free shipping to where I live, Thailand.
Anyways, problem solved, I feel so relieved. And I actually learnt a lot today. First, that I should have been more thorough from the start and check my bike instead of relying on online information, even though it came from the manufacturer. But more importantly, my mistake may well turn out to be positive, as I realised through this experience that the industry is shifting from 15 to 12 nowadays. So it means that I'll be able to keep my wheels even if I change my bike, which would have raised another problem otherwise if I had ordered a 15x100 fork thru axle from the outset.
Hope my mistake and your advice can help others now
