if you read the “90 day extension “, it says “this is not an extension of stay”. Your extension of stay is valid until April 30, 2022.
On your last day of employment, cancel your work permit, and then cancel your extension of stay based on employment. You have until 11:59pm the same day to leave Thailand before going on overstay .
You may be able to get the extension of stay cancelled in advanced.
Most immigration offices require a person to get a 30 day extension of stay based on tourism before being allowed to get a 60 day extension of stay based on unable to travel due to COVID-19 🙂