I agree with your differentiation of tithes and offerings.
However is it possible that "tithe" means a "contractual recurring percentage which is most often a tenth in scripture?"
Is it possible that its theologically argued that the "tithe" in its various percentages will add up to be more than twenty percent in Mosaic law?
However I will agree that a "tenth" is the "primary percentage that was observed in the OT" before and during Mosaic Law. Abram "tithed a tenth" of the spoils to Melchizedek, Jacob promised a “tenth” to the Lord of all that he had, and the actual “storehouse percentage in Mosaic Law was indeed a “tenth.”
I believe that “tithing” is simply a “portion that you can commit to giving on a recurring basis (daily, weekly, monthly etc...) I believe that what separates "tithing" and "offerings" is "commitment." Tithes are contractual, and offerings are spontaneous, or circumstantially strategic.
Do you feel that the NT communicates a “contractual recurring amount?” After Christ fulfilled the Mosaic Contract upon the cross, do you feel that the early church ever endorsed a “mandated recurring tenth?”
Is it possible that a “local church today can require voluntary membership to tithe a percentage” and would it be honorable to “commit to a contract with the local body as the Spirit leads?”