The War Goddess's Mechanic

Ethan, a top Earth mech engineer in his past life, is reborn as a worthless F-grade deserter in a bug-ravaged star system. To survive, he marries valkyrie Selene and endures mockery from mighty clans. Forging mech David with superb skills, he shines in tournaments, battles brutal aliens, defends humanity against all odds and wins true love.