What are the options for extending my stay in Thailand with an arrival visa waiver before switching to a Non O visa?

any way you exit / re-enter would work for activating that eVisa once you get it approved. Book round trip to KL or Singapore, fly in, turn right around and fly back out, present the eVisa PDF when you go to stamp in here in Thailand. You'll be fine.

You don't even have to stamp in to Malaysia (you can stay in the transit area if your flights in and out are tight), just show your boarding pass when you stamp back in to thailand. AND Singapore doesn't stamp people in anymore I think

OR

Bounce out and back by land, BE prepared for them to not have seen an eVisa before though at land crossings because no nearby country's thai consulate uses the system.

If you do go the embassy route avoid Cambodia there the slowest embassy of them all fastest are Laos and Singapore but check as most of the embassy are fully booked until august especially in Vietnam

IF you already applied for and got issued the Non Immigrant O visa, you can fly out to any country and come back and use that visa to enter Thailand.

You can not get twice a 30 day Extension of Stay based on tourism, those are only issued once per entry.