First experiences can be cruel.
As I've posted previously, the rules for Thai visa extensions are available online.
Also, available at mine and probably yours, local TIO, in Eglish and Thai.
My advice for your next year application:
1. Obtain the current, a month or so before you apply, "rules" from the government site and local TIO. Some "rules", financial records', requirements are 3 months or one year, PPPPP. Others obtained from your local district office, they will have an English speaker.
2. Compare them and any differences, clarify with your local TIO. Take both versions. The government/Bangkok "rules" prevail.
Why, because your application is sent and decided upon, by Bangkok, not your local TIO. Their responsible is that your application, for your extension, provides the requested information in the agreed standards.
3. As the decision affects you personally, more than your wife, you need to understand the rules and supply the previously agreed documentation as required.
4. English is an official language in Thailand, the TIO has to employ an English-speaking person. At my TIO the most proficient person is the senior administrator. The others can speak some, but may rely on the Thai fallback of shouting.
5. Keep a copy of all the requirements for next year. A % may of course need a revision.
Or:
6. Utilise an agent. You will of course have to provide them the same correct information.
It is, as many in Thailand repeat, "up to you."
Good luck next year.