What steps can I take to appeal an immigration extension decision in Thailand?

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Well in fact your research was wrong and you tried to talk to the immigration as you know better than them - no wonder they where not “kind” - but they did everything right - the 60 days can be granted after the first 30 days EXTENSION has been granted

Sounds like you didn't enter with a visa but with visa exempt (first 30 days). Rules stipulate that before being eligible for the covid extension (max 60 days) you must first use the normal 30 days extension available to you (second 30 days). Once you have used the initial extension (30 days) you may then APPLY for an extension of stay due to being unable to travel (up to 60 days). In most immigration offices (if not all by now) your application goes under consideration for a period of between 15 and 30 days. Once that under consideration period comes to an end, you return to the same immigration office and have the remainder added to your passport (45 days or 30 days as may be the case). Your research is flawed. The immigration officers have done you a big favor. When you return to get the remainder of your stay inked in, you may want to bring some doughnuts by way of an apology.

There is no Extension of Stay based on future work. If you did not find a new job or your employer can not get the documents ready on time, the option is to go abroad and apply for a Single Entry Tourist Visa which gives you 60 days or come back again without visa for 30 days and hope you get the documents from your employer with minimum 15 days on your stay to convert to a Non Immigrant B visa.

Unfortunately your information was very wrong.

The advice given here is accurate.

And just for information: The price to apply for an Extension of Stay is 1,900 Thb.

Price is the same as it is the price to apply, it does not depend on the term of extension, you can get 1 year, 90 days, 60 days, 30 days, or even 7 days to leave the country.

The under consideration period is part of the maximum amount of days you can get on this application and is used because the officer can not decide himself and have to sent it to his superiors to consider your application.

Unfortunately, the Immigration officer was right, and you understood wrong.

That series of extensions you got is exactly what it should be.

You got a 30 days tourism extension first, then got 60 days Covid extension now. You will go back to the Immigration office after 15 days of under-consideration, you would be stamped for the rest of 60 days.

You seemed to enter Thailand on a visa-exempt scheme that gives you 30 days on entry without any actual visa. This is eligible for a "30 days tourism extension", not for 60 days. So IO gave you this extension on your first application. Under normal circumstances, that's all you can get and you are done at this point.

But, currently, still, a temporary-extension-based-on-being-unable-to-travel-due-to-Covid-situation program is running, IO kindly gave you this extension for you even though you wrote the wrong reason, and even IO could easily refuse your application for the reason you wrote since this extension approval is totally at the discretion of the office.

The covid extension is for 60 days, a two-step application. After the application is approved, you would be stamped for 15 days to 1 month under consideration. You will go back to the same immigration office on the day of the stamp, then you will be stamped for the rest of the days (60 days - under-consideration period) without the fee.

I'd be thankful if I was you that your local IO gave you those extensions after you tried to "negotiate".