If the evaporator’s surface area is not covered at least 85% with frost then you are correct system needs more R12 refrigerant. You are not correct in saying “but it is R-12 refrigerant, which is no longer allowed in the US.” The Clean Air Act prevents the manufacture of CFC Freon refrigerants, there is plenty of it available because it is recycled. There is a couple of replacement refrigerants like 409a but are not friendly in Adler Barbour units in warm climates but seem OK north of Latitude 27N.

Another incorrect statement “For the AC in my car, it means a change out of nearly every part.” No, only drain oil from compressor and add Ester oil and 134a refrigerant.

If evaporator frost covers 90% or more of evaporator’s surface and there is good airflow through condenser problem is not with refrigeration.

My advice is if you confirm refrigerant is low and leak can not be found and fixed replace unit. Leaks in evaporator can not be permanently repaired.