Is there a Latin, or English, phrase or acronym for "in summary"?
Is there something similar to TLDR that can be used in professional emails and messages?
I think phrases or labels like In short or In brief may serve better, but if you insist on Latin, in nuce means “in a nutshell; briefly stated”.
Is there something similar to TLDR that can be used in professional emails and messages?
Depending on context and content, abstract or executive summary could be appropriate headings, especially if placed at the beginning of the document. Summary and conclusion are often used at the end of a document.