Is there a way to "reset" all karmic conditions with immediate effect? It is my basic understanding that certain actions such as killing one's parents or harming the Buddha has immediate karmic reactions. Are there actions that have immediate "wholesome" outcomes or ones that allow you to start with a clean slate?
There is a very famous simile in the Lonaphala Sutta: The Salt Crystal. An excerpt:

&quot;There is the case where a trifling evil deed done by a certain individual takes him to hell. There is the case where the very same sort of trifling deed done by another individual is experienced in the here &amp; now, and for the most part barely appears for a moment.

The Buddha gives a simile of dropping a salt crystal into a cup and asking if the water was fit to drink and, of course, the answer is no. It would be too salty due to the small amount of water in the cup. The Buddha then asks if a salt crystal were to be dropped into the  Ganges River if it would be fit to drink and, or course, the answer is yes because of the large amount of water in the river.
So the same action does not necessarily lead to the same effect nor to the same length of time for the ripening of the effect. The Buddha then gave some very helpful details of what type of people may experience lessor effects of their bad karma.

Now, a trifling evil act done by what sort of individual takes him to hell? There is the case where a certain individual is undeveloped in the body, 2 undeveloped in virtue, undeveloped in mind [i.e., painful feelings can invade the mind and stay there], undeveloped in discernment: restricted, small-hearted, dwelling with suffering.   A trifling evil act done by this sort of individual takes him to hell.
'Now, a trifling evil act done by what sort of individual is experienced in the here &amp; now, and for the most part barely appears for a moment? There is the case where a certain individual is developed in the body,[3] developed in virtue, developed in mind [i.e., painful feelings cannot invade the mind and stay there], developed in discernment: unrestricted, large-hearted, dwelling with the immeasurable. A trifling evil act done by this sort of individual is experienced in the here &amp; now, and for the most part barely appears for a moment.

So while there may not be an immediate reset button to avoid the effects of past karmic actions, there is the idea of performing wholesome actions now and building merit to dilute &quot;the salt&quot; of past bad deeds and to become the type of person who experiences lessor effects of bad karma. Some people concentrate on building merit by giving offerings (dana), by practicing virtue (sila), and by practicing meditation (bhavana) in this effort.