Your results are already very good for a homemade tracker. To improve further, the main factors are tracking accuracy, stability, and stacking workflow.

At 135mm, even small tracking or polar-alignment errors will show. A barn-door tracker can reduce motion within one exposure, but keeping every frame aligned pixel-perfect across multiple exposures is harder. That “blur” in the stack may be misalignment rather than true motion blur, especially if it’s worse near the edges. Correct lens distortion before stacking and make sure the stars are aligned properly.

Other practical improvements:

• Use shorter exposures if stars still elongate.
• Improve tracker alignment to the Earth’s axis as much as possible.
• Increase stability: avoid wind, and make the tripod/tracker more rigid.
• Use a remote shutter release to reduce vibration.
• For cameras with mirror slap, use mirror lock-up; with mirrorless this is less of an issue.

A wider lens will hide tracking errors more easily than 135mm, while a longer lens will make them more obvious. Light pollution from nearby lamps will also limit image quality, so darker skies will help overall contrast and detail.