Is there any long term dangers of having a constant regular sinus rythym, yet slightly tachycardic heart rate?

Probably not. There is a tachycardia-induced cardiomyopathy which is occasionally seen in people with a chronic tachycardia. The exact rate necessary to produce the problem is not clear but it would be less likely to occur with a "mild" tachycardia. When there is tachycardia, though, we always look for the cause. If your thyroid gland is overactive for instance that needs to be treated.
Review causes. The long term dangers arise from the cause rather than sinus tachycardia itself. Some causes include deconditioning due to lack of physical activity, being overweight, having anemia, nicotine/ excessive alcohol/ excessive caffeine, hyperactive thyroid, chronic lung disease or chronic pain. Please see if any these apply to you and talk to your doctor about it.