A lot still unclear but I guess there's still time. Will you be able to check in for a flight to Thailand without having completed it, or is it only checked at passport control in Thailand? Can you complete it after arrival, like you could with the paper TM6?
I've usually done it the other way round. Get the new Extension, then my HR officer goes to MOL to get the new WP stamp (without me having to be there!).
If you're planning to get a Retirement extension, it's best to get the Non Imm O visa in your home country before you come to Thailand, then extend here. Same if you're married to a Thai. Getting any type of short-term visa aimed at tourists isn't recommended.