We have to ask who is ultimately sovereign. If God wants you saved, can you stop Him?  If yes, YOU are ultimately sovereign in that matter.  If no, then He is indeed sovereign. And it's clear from scripture that He knows his own sheep and "no one [including us] can take them out of my hand."
We are indeed presented with a choice, we are not automatons, but in our state of sin we will only ever chose sin. Grace provides us with the choice not to sin. “We love him because he first loved us”. We are “saved by grace through faith” and this is the “free gift of God”. He knows his sheep and he will “keep” them to the end. 

It's in light of that belief that we then approach individual passages like Heb. 6 and ask "what does 'have tasted' mean?  Firstly, it doesn't mean full, saving faith. It means those who have experienced and seen aspects of the visible church but not themselves truly entered in. It also means literally tasted, as in communion. Further, when contextualized, the specific audience/recipients of the letter were of the generation that crucified the Christ and were forgiven for it. To reject him again and return to the old covenant is tantamount to crucifying him again (striking the Rock twice, as Moses typologically shows us). Hebrews divorced from its first century context and the coming destruction in 70AD has a face value reading of being universally applied and presents all kinds of wild implications. 

I enjoy these debates and discussions and enjoy learning from and being sharpened by my brothers, especially those in other traditions. That said, it seems to me that this particular discussion comes down to fundamental differences in Roman Catholic and Protestant soteriology and I don’t think that either of us will budge much because of a Nostr thread.