If I haven't prayed or participated in religion that much, but I haven't done any major sins either, and I still believe in its tenets and do wish to change, how may I still transform my life for the better?
Bismihi ta'ala
As you know Islam encourages a positive and healthy spiritual and moral lifestyle, where one of the most important elements of life is building a relationship with our Creator, Almighty God. This can be done by offering a minimum of a few minutes a day and devoting it to praying to the Almighty.
What God wants from us is not that much, and all the benefit will come back to us.
Avoiding sin is one side of religion, and indeed an important side, but so is participation. A Muslim must try to practice as much of Islam as possible, furthering his or her spiritual progress.
Change will occur when you put your mind to feeling God in your practical life. When you surround yourself with Allah ta'ala, you will automatically turn to following His commands. You will feel an inner strength, and this will make your go through the transform that you need.
Living an Islamic lifestyle will give you the enlightenment that you need as a human being, and it will fulfill your purpose in this dunya.
I would encourage you to read books on Islamic Akhlaq, like certain books written by the late Shaheed Ayatullah Dastghayb.
Wassalam