"Tend to" is similar to other compound verbs (like "go to eat" or "open to receive"). The form is tend + (verb infinitive).:
They tend to play outdoors.
He tends to make outrageous comments.
She tends to walk into people because she's busy texting.
and so on.
Tending is not as often used, possibly because "tend" by itself already indicates a trend. If you look up the use of "tending" you'll far more often find its other definition as the gerund form meaning "to give your attention to and take care of". It's more natural to simply say "tend to become" rather than "tending to become".
"Tend to be better" makes sense, but it's not a natural expression. "Tend to improve" is the more common way to say this, along with similar phrases (tend to progress, tend to increase, etc.). Examples:
Over time, their performance on proficiency tests tends to improve.
Over the past few weeks, the trainees have tended to improve their hand-eye coordination.
Last month the unemployment rate tended to increase, but this month it's been flat.