Since gentiles will be those who are entering in, when they fail to be born any longer, then the family is complete.
I look at "when the fullness of Gentiles come in" as the completion of an intended plan whereas you see it as the beginning of their failure - wouldn't your interpretation have been better captured with a "when the Gentiles no longer come in, then Israel will be saved"?
And I guess that stems from our very different understanding of what causes one to be born of God - whether it is God Himself birthing His children of promise or if man causes himself to be born by his faith. That in itself is a larger discussion.
But let's trace God's plan here where God determines to conclude all in unbelief so that He can have mercy on all. The gentiles first did not believe God but now are receiving mercy and are reconciled. Likewise, Israel is now not believing God but will receive mercy too in the future. And their reconciliation is the end of the world. That's all I see it for.
But you add more to this - you're saying that the Gentiles that are already reconciled now and who are already receiving mercy will again fall into unbelief and these gentile branches will be cut off and then Israel will receive mercy. So the end of the world cannot yet come because now the cut off Gentiles will again have to be shown mercy for their unbelief and will need to be grafted in again and be reconciled all over again. Where do you find evidence for this additional loop of reconciliation when no such thing is mentioned in Rom 11?
but toward thee (gentiles), goodness, if thou (gentiles) continue in his goodness: otherwise thou (gentiles) also shalt be cut off.
This is a principle being stated - with both the IF and the ELSE. It does not explicitly say what will transpire here, does it? That's something you're interpreting from elsewhere (and which I've argued against above), but to say this is the verse which concludes that the Gentiles will no longer believe and be cut off is at best ignorant of interpretative principles. Do you likewise read say Rom 2:26 and conclude that the gentiles have indeed kept the righteousness of the law - or do you read it merely as a hypothetical to make an argument for a principle?