Is a person who properly and sincerely converted to Judaism many years ago, regularly attended and was active in a synagogue for a number of years, later became mostly nonobservant, and then came to believe, more or less, in pantheism still a Jew?

Yes, he is still 100% Jewish. As soon as one converts, he is totally Jewish, and God willed that there be "no way out". Like a king whose teenage son doesn't want to be king because he doesn't appreciate the advantages, so the father keeps the role for his son, so that when he matures and understands, he will have the honor at his disposal. BTW, I suggest you speak to a Hassidic rabbi who may believe in pantheism or panantheism which is somewhat compatible to Judaism.