The first mention I find in the WTS publications is 1958. Although the WTS says the Index goes from 1930 to 1985, nothing appears before 1958 a search only found by using the phrase "spiritual paradise" not in the index. In 2015 there was a clarification of the phrase "Of course, we should not conclude that the terms “spiritual paradise” and “spiritual temple” are the same. The spiritual temple is God’s arrangement for true worship."
2022 EXPRESSION EXPLAINED: “Spiritual paradise” refers to the secure environment in which we worship Jehovah in unity. In this environment, we have an abundance of spiritual food that is free from religious lies and we have plenty of satisfying work to do in preaching the good news of God’s Kingdom. We enjoy a close relationship with Jehovah, and we live in peace with our loving brothers and sisters, who help us to endure life’s challenges with joy. We enter the spiritual paradise when we begin worshipping Jehovah in the right way and when we do our best to imitate him.
2015 The expression “spiritual paradise” has become a part of our theocratic vocabulary. It describes our unique, spiritually rich environment, or condition, which allows us to enjoy peace with God and with our brothers. Of course, we should not conclude that the terms “spiritual paradise” and “spiritual temple” are the same. The spiritual temple is God’s arrangement for true worship. The spiritual paradise serves to identify clearly those who have God’s approval and who are today serving him at his spiritual temple.
1983 “What do you mean by a spiritual paradise?” you ask. Put simply: An organization of Christians who have made over their personalities to conform to God’s requirements and who are united in his true worship may be said to be in a spiritual paradise.
(oldest in WOL) 1958 Thursday afternoon the conventioners waited with intense interest to hear Brother Knorr speak on “Maintaining Our Spiritual Paradise.” \... Knorr told the vast audience of 145,488 persons that when the holy spirit was poured out on Christ’s disciples at Pentecost, making them spiritual sons of God, “a spiritual paradise had then been established.” (Notice that Knorr refers to spiritual sons of God—at that time only the anointed had the designation)