I already explained to you that John 3:16 is not iffy salvation. The word "should" subjunctive mood expresses a possibility or a consequence which results if a condition is met. In the case of John 3:16 the condition is “whoever believes in Him.” When that condition is met, two things happen. The person shall not perish, but receive eternal life and half or more of the English translations of John 3:16 do not use the word should. The NASB and NIV read, “whoever believes in Him shall not perish, but have eternal life.” The NET Bible, LEB, and HCSB have the same translation except they use "will" instead of shall (will not perish). In John 3:18, even in the KJV, we read - He that believeth on him is not condemned.. Hmm.. so why doesn't it read, "should not be condemned?" So much for your iffy salvation.
Saving belief in Christ continues (in the present tense) and is not some shallow, temporary belief that has no root, produces no fruit and withers away, as we see in the 2nd type of soil in the parable of the sower. Unlike saving belief, temporary, shallow belief is not rooted in a regenerate heart. How can no depth of earth, no root, no moisture, no fruit, represent saving belief? It doesn't. Also the same Greek word for believe "pisteuo" is used in James 2:19, in which we read that the demons believe "mental assent" that "there is one God," but they are not saved.
Now even though this shallow ground hearer in Luke 8:13 is said to have "believed," yet he is never said to have been "saved." How do we know that the shallow ground hearer was never actually "saved"? I will explain the reasons.
First, his heart condition is contrasted with that of the "good ground" hearer in the 4th soil, who's heart was "good" and "honest." Thus, his heart was not "good," being like the soil to which it corresponds, being "shallow" or "rocky," lacking sufficient depth. Such soil represents a sinner not properly prepared in heart. People who "believe" and "rejoice" at the preaching of the gospel without a prepared heart, and without a good and honest heart, and without having "root" in themselves, do not experience real salvation.
John has portrayed people who "believe" (at least to some level) but are clearly not saved. There is a stage in the progress of belief in Jesus which "falls short of firmly rooted and established belief resulting in salvation." As we see in John 2:23-25, in which their belief was superficial in nature and Jesus would not entrust/commit Himself to them.
Also, in John 8:31-59, where the Jews who were said to have "believed in him" turn out to be slaves to sin, indifferent to the words of Jesus’, children of the devil, liars, accused Jesus of having a demon and were guilty of setting out to stone and kill the one they have professed to believe in. We can see at best, these Jews believed in Him (based on their own misconceptions and expectations) of Jesus, yet upon gaining further knowledge about Jesus through His words, we see they did not truly "believe unto salvation" and become children of God (John 1:12; 3:18) but were instead children of the devil.
IN CONTRAST TO - Mark 4:8 - But other seed fell on good ground and yielded a crop that sprang up, increased and produced: some thirtyfold, some sixty, and some a hundred. Luke 8:15 says, But the ones that fell on the good ground are those who, having heard the word with a noble and good heart, keep it and bear fruit with patience. So the rocky soil represents a person not properly prepared in heart and the seed planted ends up with a lack of "root" (lack of being firmly planted, or established) and good soil represents a person properly prepared in heart who having heard the word with a noble and good heart, keeps it and bears fruit with patience.