https://www.amazon.com/Rebellion-Ailon-Thaddeus-Marcell-Chronicles-ebook/dp/B07KWHD3B5
Thaddeus Marcell is feeling really burned out after his mission to rescue Adelia near the Waverly system. Deciding to go on a sabbatical to avoid combat, heal from his wounds, and reflect on his life, he takes on a false name and travels to an enslaved planet called Ailon, quietly volunteering with a medical charity there.
However, he soon finds himself in the middle of an insurrection. The people of Ailon have formed a rebellion to remove their oppressive regime and free the enslaved population! But this new rebellion is woefully under-trained and under-prepared for the war they are about to begin.
Now, Thad must make a difficult choice. Should he leave Ailon behind, continuing his sabbatical elsewhere while dooming the new Ailonian rebels to failure? Or should he intervene, using his skills as a mercenary commander to lead them to victory? It's complicated, because he harbors a dark secret about Ailon: its enslavement was his fault!