First post on the site and I have not read all the posts so forgive if this has been covered multiple times .
Single entry tourist visa can be extended 30 days , on day of arrival or any days within the time span of the visit . You do not need to wait til the final few days
So do an extension at your convenience at a time that suits you . Just flew in yesterday , just done my extension , 88 days not having to worry about anything visa related .
TLDR : Answer Summary
A single entry tourist visa in Thailand can be extended for an additional 30 days, and the extension can be done at any point during the stay, not necessarily just before the visa expires. This flexibility allows travelers to manage their time without worrying about visa issues.