If so, the route to become an FI does not involve taking CPL/ATPL theory
I think that statement needs a LOT more clarification before recommending it as a possible route.
I doubt very much that instructors without the CPL level of knowledge will be accepted by EASA...let me check Part-FCL....
FCL.915.FI 2(b) For an FI(A) it will be a CPL or PPL with CPL theory.
Unless you want LAFI only...in which case just the LAPL would be needed.
So basically if you go down a route other than JAR-FCL FI you are taking a huge gamble as to the conversion requirements from the NAA to EASA and may well find yourself having to do it all over again at worst case.
Tread carefully friends!