I wouldn't say that the words of scripture are insubstantial, because "the word of God is living and active, sharper than any two-edged sword, piercing to the division of soul and of spirit, of joints and of marrow, and discerning the thoughts and intentions of the heart". (Hebrews 4:12 ESV) The Christian does have firsthand experience of God when encountering him in scripture, because we have the Holy Spirit, of whom Jesus said "the Helper, the Holy Spirit, whom the Father will send in my name, he will teach you all things and bring to your remembrance all that I have said to you." (John 14:26 ESV) So in scripture we have a solid foundation for truth, understood through the teaching of the Holy Spirit.

In contrast, none of the methods you mentioned above have certainty of the truth of the revelation given. Meditation can mean a broad range of things, and thinking deeply on God's word is commended in scripture, but just trying to come up with ideas of God from your own mind will quickly lead you astray. Hallucinogens are often illegal, and can obscure your senses in unhelpful ways, but everything seen in such hallucinations is likely to come from your own mind. Sex within marriage is a good thing, but it is not a path to see God; thinking that could lead you to worship sex or the person you have sex with, both of which would be wrong.