Is it okay to overstay in Thailand for 6 days if I pay the fine, or should I get a visa extension instead?

I see no reason or justification why you can't get an extension of stay OR an extension of stay denied stamp. I'm closing comments.

No issue beside if the police check and you on overstay you get 3 years ban from comming again

Get the visa

Covid visa would be cheaper and legal

A 60 day extension of stay based on unable to travel due to COVID-19 is 1,900 THB. The last day they are selling it is November 26th.

An overstay fine is 500 THB a day. Being six days overstay is 6 * 500, which is 3,000 THB.

Get the 60 day extension of stay based on unable to travel due to COVID-19.

If you will be staying after the 60 day extension of stay based on unable to travel due to COVID-19 is being sold, you can get a 7 day extension of stay denied stamp for 1,900 THB. GO on your last "admitted to" day, the extension of stay denied stamp is NOT added on to your existing "admitted to" stamp.

Overstaying is a crime. Read my post at 3/permalink/7/