Can I enter Thailand on a tourist visa while waiting for my non-B visa approval?

Land borders are closed and flight prices are going through the roof at the moment you might want to change your flight

Even applying for Non-B in the country required me to run back and forth multiple times until it got approved. You have low chance of getting approved abroad.

I like your idea of going into the country with tourist visa, because it's the same visa category and change it to Non-B later in the country. But, you must reschedule your flight because it requires 5 days to get Tourist visa in Cambodia.

𝗜𝘀 𝗶𝘁 𝗮𝗻 𝗼𝗽𝘁𝗶𝗼𝗻 𝗳𝗼𝗿 𝗺𝗲 𝘁𝗼 𝗴𝗼 𝘁𝗼 𝗧𝗵𝗮𝗶𝗹𝗮𝗻𝗱 𝗼𝗻 𝗮 𝘁𝗼𝘂𝗿𝗶𝘀𝘁 𝘃𝗶𝘀𝗮 𝘄𝗵𝗶𝗹𝗲 𝘄𝗮𝗶𝘁𝗶𝗻𝗴 𝗳𝗼𝗿 𝗺𝘆 𝗻𝗼𝗻 𝗕 𝘁𝗼 𝗯𝗲 𝗮𝗽𝗽𝗿𝗼𝘃𝗲𝗱

Nope not an option for you to enter thailand on ANY other visa while that one is pending 😕

BUT with that being said,

I believe what you're actually asking is "can I go to thailand on a FREE STAMP 60 day "visa exempt entry" <- which is not a visa at all, just a free entry stamp.. and that answer is YES, you can

IF your visa doesn't get approved before your flight you could indeed fly back on 60 day visa exempt entry (depending on how many of them you have been here on recently)

and

once the Visa is approved then bounce out/back to activate it

If you'd been here a while on multiple free stamp visa exempt entries, you might have an issue comin' back in free stamp. Even telling them you have a Non-B application pending isn't gonna cut any ice letting you in. Who knows you might skate thru, but you need to be prepared to be questioned depending on your previous entry/stay history