Should I continue to combat obsessive thoughts with the same method currently employed, despite the persistence of some doubts about purity after seeking refuge with Allah and spitting (lightly, to the left)?

As for visions, we have no knowledge of their interpretation. As for whisperings (waswasah), there is no better cure for them than to disregard them, pay no attention to them, and continue to ignore them until one is cured of them. Do not heed what Satan adorns, suggesting that these whisperings have a basis, unless you attain absolute certainty that impurity has occurred or an obligatory ritual bath has been omitted. You will not feel complete peace until you are entirely rid of them.