Seems like a temperature issue to me.

Short parts: Each layer is finished quickly, so the plastic stays warm, and layers fuse strongly before cooling.

Long parts: Each layer takes much longer to complete, so by the time the nozzle returns, the previous line has already cooled too much, leading to poor interlayer adhesion.

Try those one by one:

• Raise nozzle temperature.

• Raise hotplate temperature.

• Reduce part cooling fan speed, or disable it after the first few layers.

• If you have an enclosure, raise the enclosure temperature.

Some other things to try:

• Slow print speed. That makes more consistent extrusion, especially on long straight runs.

• Increase flow/extrusion multiplier slightly (e.g. +2–4%) to compensate for under-extrusion on long lines.

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This one is off topic, but you mentioned a “debugging” flowchart / q/a for plants or gardening and thats exactly what I am looking for. Do you have a link (or book recommendation) to such a resource?