I wouldn’t deny that’s it’s possible to boast for the reasons you specify, Jeff, but I just haven’t run into it personally–believers actually boasting of the fact that they did something that earned them some praise and respect by simply because they chose to accept grace, to say ‘yes’ to God. But if it happens, it can’t really be evidence against synergism per se, for then (by parity of reasoning) not boasting would have to be evidence FOR synergism, and I wouldn’t claim that either. And then the same is true for monergistic approaches. You have boasting (of a more Israeli sort) on that side, like “Hey, God chose ME. I must be swell. I’m special. You’re not. I’m superior to you.” You definitely have that going on. But not every OT Jew boasted of God’s having chosen the Jews. But some did.
We’re good at perverting our relationship with God however we say it’s to be understood. Not sure that in itself reflects upon the truth of how we say it’s to be understood.
Tom