That is what that passage says. You’re trying to make it say a person can lose their salvation and that’s impossible under the new covenant.
So, you are obviously a Calvinist and that explains some things while your belief in preterism explains the rest.
Hebrews 6:4 For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come, 6 If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
Explain how any of "those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, 5 And have tasted the good word of God, and the powers of the world to come" could possibly not be saved? If that isn't a description of a saved person, then I don't know what is.
They were observing repentance through the old covenant. When the validity of that covenant ended and they rejected Jesus and the new covenant, it was impossible to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame.
You don't seem to understand how salvation works. It has always worked the same in the sense that it has always been by grace through faith and not by works. So, anyone who repented in old covenant times was saved. That passage is talking about the possibility of people who were saved to fall away.
Paul wasn’t told who was and who wasn’t a tare. Not every single Jew was a tare. The Son of man sowed first in that old covenant field then Satan sowed the tares. Some Jews were tares some were not.
Have you actually read Romans 11? He differentiated between the elect remnant who were saved and the rest who were blinded (Romans 11:5-7). In Romans 11:11-14 he was only referring to those who were blinded and said that they stumbled (only unbelievers stumbled, not believers) but did not fall and then said he hoped to help save some of those who stumbled, which were those who were blinded in order for the gospel to go to the Gentiles so that they would then be provoked to jealousy by the Gentiles.
Exactly some might be saved but not all as some were tares
All of those that Paul was talking about in Romans 11:11-14 were tares because the context involved only those who were blinded and not the remnant who were already saved. Paul said he hoped to lead some of those who were blinded to salvation. They were not blinded permanently, but only temporarily while the gospel went to the Gentiles. God's plan for them was to blind them since it was His plan for them to remain in unbelief and reject His Son so that He would make His planned sacrifice on the cross and it was His plan for the gospel to go to the Gentiles so that those blinded Israelites would then be provoked to jealousy and want to be saved like the Gentiles were. That's why Paul had the hope to lead some of them to salvation because he understood God's plan for them.