From a materials of construction perspective, this is a tricky mixture to deal with, especially at 80 degrees C and higher. Phosphoric acid is less corrosive than sulfuric acid. Pure phosphoric acid has no oxidizing power but commercial phosphoric acid contains impurities such as fluorides and chlorides that can significantly increase its corrosivity. The corrosivity of sulfuric acid depends on many factors including temperature, concentration, the presence of oxidizing or reducing impurities, velocity effects, and solids in suspension. It is usually not wise to select materials of construction for sulfuric acid handling equipment based only on published corrosion data since corrosion by sulfuric acid is a complex phenomenon. Small differences in impurities, velocity, or concentration can significantly impact the corrosion rate. Halides generally increase corrosion while aeration or the presence of oxidizing agents usually increases the corrosion rate of non-ferrous materials  and reduces the corrosion rates of stainless steel alloys. I strongly recommend laboratory corrosion studies be run on your specific stream as a part of your material of construction selection process.

In your question, you said you are experiencing problems with 304 stainless steel at the welds. If this is the case, you might consider moving to 304L stainless steel. Low carbon versions of austenitic stainless steels like 304L are designed to eliminate problems associated with carbide precipitation and chromium depletion at welds. If 304L doesn't work, try gradually moving up to a higher alloy. Possible candidate materials in order of generally increasing corrosion resistance are: 316L stainless steel, 20-type alloys like 20Cb-3, higher chromium Fe-Ni-Mo alloys like Alloy 31, and nickel-base molybdenum-chromium alloys like C-276.