I have rentals in Tarrant County and you can file a protest directly online yourself at tad.org My Property Mgr refers a tax protest service, but I opt out, since it is pretty easy to do yourself and I am an Appraiser and don't see the need to pay 25-40% for the service. Now, it may be worth it in some instances, but I have not had any trouble doing it online. Like you, I am in CA so I wouldn't go in person.
If you don't yet have an account with tad (tarrant appraisal district) you need to go to tad.org and sign up. Then you can add your property/ies to your account, with the info on your property value notice (account # and online PIN). Then you just click on the property and there is a Protest Value button and fill out the form. If I remember correctly, they also provide the valuation of other properties on which they based your valuation on. But, I don't see that option on their website right now, but their website is very slow at the moment, probably due to high traffic. But, they should provide you with that information. There are 3 reasons for protest on the form, incorrect appraisal value as compared to other properties, incorrect appraised value and/or value is unequal compared with other properties. That's why you need to know the other properties' values that they used to appraise yours.
Anyway, the hard part is getting the data since TX is a non-disclosure state, which means the sales data is not public. You can use zillow and look up sales in the neighborhood around Jan 1 (I believe that is their valuation date). Then you can change the search criteria to find similar properties to yours and if click on the individual sales and scroll down on the property info, zillow lists the date the property was listed in the mls and the list price and it lists the sale date. So, the sale price is most likely relatively close to the list price. So, that gives you a good idea of what value your property should be. Anyway, I do get mls data for the area, which helps me. Let me know if you'd like any help.
At least their is a silver lining, since the higher tax value, is reflecting that prices have gone up. But, yeah, TX prop taxes are high!