[12mm to 15mm thru axle adapter]

Interesting idea - engineering option sounds cost effective, will also look into that

[Can I use a Specialized Roubaix as a gravel bike?]

I did this on an old 26” HT I converted to “gravelbike” by adding 700cc tyre but kept the front suspension. Fitted 33mm gravel tyres and it is a very nice bike, and never felt I need wider tyres, but obviously the 100mm travel in front helps. The futureshock may have a similar impact 

I think it can work well but depends on terrain you will ride - if mostly good jeep tracks, farm and corrugated roads and endurance rides I think it is a great and cost effective option.