Dear brother Bruce,
I agree that many people will be deceived exactly because of the strong delusion, but carefully examining that verse, we see the following:
To whom the strong delusion is sent? To all people on Earth that weren’t taken in the rapture? No, only to "those who perish", those who had heard the truth in a clear and sufficient manner, but chose to reject it. They refused the truth and had pleasure in unrighteousness.
Many gentiles on Earth have not heard the true Gospel yet. And many have heard it in a way that isn't sufficiently clear or is much distorted.
So your statement that "[all] those who are not raptured, at the time of the rapture, will receive a strong delusion from God", in my humble opinion, is not accurate. Only those that heard the Gospel in a very clear and sufficient way) and purposefully chose to reject Christ are those "who perish". All the rest who are not destined to perish will not believe the strong delusion, because God will protect them from the error. These other people are not in the group of "those who perish".
This group (of those who are not raptured at the time of the rapture) might even include those who had already believed and received the truth but chose to live in a sinful and unrepentant way at the time of the rapture. They were not watching (not in terms of dates, but in terms of their spiritual state and practical lives), were not raptured but still belong to Christ and will be saved eventually.
The expression "those who perish" is specific to who the strong delusion is sent to. This expression doesn't apply to all those who are not raptured at the time of the rapture, as you generalized.
Concerning your opinion that "Gentiles tribulation saints will be rare indeed", I think it’s incompatible with Revelation 7:9. That passage clearly shows us that the Tribulation Saints are "a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues". Verses 13-15 confirm specifically that those people are indeed Tribulation Saints.
This doesn’t mean that anyone should rely on second chances and not be watchful, but after we’re gone, it’s very comforting to know that our God is a God of mercy and will gives plenty of opportunities to those gentile new believers (or perhaps left behind believers too, if that’s the case, which I don’t know) who remain on Earth (or who are born) after the rapture and are not destined to perish. If there was no hope for them, God wouldn’t bother to send so many witnesses during the Tribulation (the Two Witnesses, the 144k and even angels). These witnesses are primarily for Jews but they will reach gentiles too.