You may get better results with a higher speed and more power, because 6 inch/min = 2.5 mm/s is extraordinarily slow for a CO₂ laser on what’s basically wood.
Ruida controllers have a minimum starting speed, below which they apply only the minimum layer power. If the layer speed is below that value, there’s no power modulation at all, so the corners get burned as the machinery slows down and speeds up without compensating power adjustments.
However, large lasers don’t do well following small complex shapes, because they have relatively low accelerations. Running at a higher speed may not allow enough power modulation near the corners, because the speed can’t change rapidly enough.
Instead, convert that layout into filled shapes, then use a Fill layer, rather than Line. The laser will scan back and forth over the entire pattern, switching the beam on and off, so the whole thing gets a uniform exposure.
If you have the original layout as a JPG / PNG image, you can use that. Otherwise, converting the layout will require some redesign to broaden the fine strokes and adjust the lettering.