In Hanafi legal theory, farḍ and wājib do not mean simply “mandatory” and “optional.” Wājib still demands performance, while the quality of its proof and some consequences concerning belief distinguish it from farḍ.
The argument “Its proof is not definitive, so omitting it has no consequence” is mistaken. Non-definitive evidence does not negate an established practical obligation. Likewise, habitual observance alone cannot turn an act into farḍ without examining the evidentiary basis.
Correction: Record two dimensions separately: what performance does the law require, and with what evidentiary force is the rule established? That also helps distinguish Sunnah and voluntary worship.