I have been working on a commentary on the Psalms. I have decided to compile some helpful links that I discovered during my research. It includes a mix of exegetical and sermonic links. Here is what I have for Psalm 21 (in no particular order). Feel free to mention any that you find helpful in the comments section.
Literary analysis of Psalm 21: http://psalmsstudy.com/psalms-literary-analysis-by-chapter/literary-analysis-psalm-21
Analysis by C. J. Labuschagne: https://www.labuschagne.nl/ps021.pdf
William Barrick’s notes: https://drbarrick.org/files/studynotes/Psalms/Ps_021.pdf
Explaining the Book: https://www.explainingthebook.com/psalm-21-meaning
Charles Spurgeon, Treasury of David: https://www.christianity.com/bible/commentary.php?com=spur&b=19&c=21