Do it yourself, it's not difficult. An agent will basically charge you to do it for you and then just tell you what's on the Thai embassy website already.
As things stand today the Covid extension finishes on 30 March, so if you want to use it you need to cancel your work visa before this date then apply for this extension.
Alternatively see if they announce an extension again but this probably won't be announced until very close to 30 March itself.