I don't have the exact answer but from chemical abstract I found the following:

Liquid phase catalytic oxidation 3,​5 - xylenol

• By Lebedeva, N. V.; Koshel, S. G.; Koshel, G. N.
• From Izvestiya Vysshikh Uchebnykh Zavedenii, Khimiya i Khimicheskaya Tekhnologiya (2020), 63(3), 4-9.
• 5-​Hydroxyisophthalic acid is of practical interest in the synthesis of polymers, drugs, cosmetics. The methods for its prepn. described in the literature are characterized by a no. of significant drawbacks, which hinders the use of this compd. in the development of new materials with valuable consumer properties. In this regard, a method for the synthesis of 5-​hydroxyisophthalic acid by the oxidn. of 3, 5-​xylenol has been developed. It has been established that the hydroxyl group inhibits the oxidn. of Me groups; therefore, to carry out oxidative transformations, the hydroxyl group was replaced by acetyl.  For this, 3,​5-​xylenol was acylated with acetic anhydride in the presence of sulfuric acid. The resulting 3, 5-​dimethylphenylacetate is oxidized by atm. oxygen in an acetic acid medium in the presence of a catalytic system contg. cobalt acetals manganese and bromide ions up to 5-​acetoxyisophthalic acid. The effect of temp. on the reaction was studied and the activation energy was detd.  It was 50 ± 5 kJ​/mol that corresponds to the activation energy of oxidn. of alkylarom. hydrocarbons. The dependence of the rate of the process and the yield of the target product on the concns. of the starting compd. and the components of the catalytic system is investigated.  It has been established that carrying out the oxidn. process of 3,​5-​dimethylphenylacetate at its initial concn. of 0.75 mol​/l, temp. 110°C and cobalt (II) acetate concn. 0.1 mol​/l, sodium bromide 0.05 mol​/l, manganese (II) acetate 0.003 mol​/l provides the reaction with high rate and the yield of 5-​acetoxyisophthalic acid 70-​75​%. In these conditions, the balance expt. was carried out. 5-​Acetoxyisophthalic acid is of practical interest as a monomer for heat-​resistant polymers. To obtain 5-​hydroxyisophthalic acid, 5-​acetoxyisophthalic acid is subjected to alk. hydrolysis
• You can undergo the reaction with xylenol and measure UV/Vis of DMSO-diluted solutions

The glass bulb of a pH electrode depends on a hydrated gel outer layer to sense the hydrogen activity. Acetic anhydride dehydrates this layer. It may be possible to get some readings with an electrode that has a double salt bridge but anyway these readings are only relative and cannot be traced to aqueous solutiona.

pH is measured in terms of hydrogen ion concentration in water. Since you are using acetic anhydride and conc. sulfuric acid (no water), any reading your electrode gives is meaningless. It might actually damage the electrode. You can be sure that the mixture will be strongly acidic. When you add this mixture to water, then anhydride will slowly hydrolyze to acetic acid. Since sulfuric acid is also present, the pH will be less than 1.