Can I extend my visa in Thailand if my passport has only 5 months validity left?

Yes, local Thai Immigration Offices have granted an Extension of Stay to passports with less than 6 months validy (expires in less than 6 months)

However, any Extension of Stay granted will NOT be longer than the Expiration Date of the Passport, that is, if an Extension is for longer than the Passport's Expiration, then the Extension will only be granted for up to the Expiration Date of the Passport

The same concept of a Passport Expiring "soon" applies to a Thai Immigration Officer granting Entry into Thailand, but individual airlines may have a different policy and may or may not allow boarding on their individual flight to Thailand, so there are fewer cases for Thai Immigration Officers at Arrival and Entry into Thailand than those already in Thailand requesting an Extension of Stay, because of various individual circumstances and situations have a passport that is about to expire and have not yet renewed their passport before requesting an Extension of Stay at the local Thai Immigration Office....

This is an excerpt from 1 of 2 Thai Immigration "Field Manuals" publicly available for download.

This specific Thai Immigration "Field Manual" is สตม108 / imm108

Note that the legal basis for such a policy is rooted in the Thai Immigration Act 2522 (1979) itself

I was once given a 6-month extension right up until the last day of the expiry of the passport which had 6 months left when I applied for a one-year extension.

You should be able to, but you will know for sure when you try. Report back, please. Shouldnt be a problem

a quick note: It is not a "visa extension". A visa cannot get extended. A 30-days touristic extension is the extension of the "stay permit"

I got a new passport within 10 days from the US embassy in Bangkok. Just saying.

MAybe, case by case, no harm in trying

Yes you can.

They will only give you upto the end of your passport ie if you are saying that your passport has only 5 months left then they will only give you upto the end date

Best if you renew your passport? It should have 6mths validity on it.

No. You are required to have six months remaining.

No, get a new passport.

Yes. Not an issue.