@Marcus **************
, you wouldn't have any problem with leaving Thailand, letting your extension expire, and coming back on Tourist status without canceling your current extension before you leave.
You would face some problems when you will apply for a new long-term visa or extension inside Thailand if you don't cancel your current extension officially.
That means, you just come back to Thailand only as a tourist and wouldn't apply for any other type of visa/extension in your future, you can just leave and forget about it.