Can I get a 7-day visa extension before my flight, and enter Thailand as a tourist later?

As people mentioned, that application denied stamp (not an extension anymore) is UP TO 7 days starting the day of application. It doesn’t have to be 7 days always and number of days granted is at the discretion of the officer. Be careful with the calendar, not to be lack of days even with that stamp. Bring your departing ticket on paper as proof to leave, which may help to make the possibility to get 7 days higher.

I don’t think you would have any Issue when you enter Thailand a bit later, but it’d be good to be prepared.

You can apply for an extension that you do not qualify for. Do this on either of the two last days (hoping these aren't both weekend days). You'll get a 7 day "extension denied" stamp.

If I'm correct, you can cancel your WP and extension ahead of time regardless; all that matters is that the application to get 7 days extra is done on the last two days.

There are no extensions available to you, but, you can apply/buy any extension that will be denied, and be given a 7 day stamp to leave the country, you must leave. Typically this stamp is dated from the day you apply for the extension. Your local office may vary on this rule, go ask them, bring your passport. As James mentioned you need to cancel your work permit and subsequently your current extension, this can be done in advance with a letter from your employer, and your extension permit to stay date will be adjusted accordingly to the last day worked.

Be sure to cancel your current Non B extension of stay as working And your work permit before you leave Thailand.

You can Ask your local Thai Immigration office for a 30 day extension of stay, which will be DENIED and given an order to LEAVE Thailand within 7 days FROM the date you apply for the extension. Time your visit to the Thai Immigration office carefully.

As for your return to Thailand in a few weeks, your allowed entry with a tourist visa will be likely questioned by Thai Immigration Officers upon arrival. Be prepared to show PROOF of your flight departing Thailand within the 60 days of your arrival if on a tourist visa, or 30 days if you enter Visa Exempt Entry.

Along with 20,000 Thai baht in cash or equivalent foreign currency.

And a booking for your initial days Stay in Thailand.

Suggest you obtain the METV multiple entry tourist visa in your home country before you fly to Thailand. With planning it can get you up to 9 months stay in Thailand with a few simple LAND BORDER BOUNCE entries.

Good Luck 🍀