it does also take an actor to sell the vision of a character. an unconvincing portrayal would make the vision seem more like a farce or caricature and if anything undermines the morals

In part, yes of course. One thing that I respect a lot about Roddenberry was that when Isaac Asimov gave Star Trek a "bad review" publicly, he reached out and asked for feedback. They ended up becoming friends and Asimov helped advise him on the character development of Kirk and Spock's friendship that became central to the original series.

But, if you watch the first season of TNG, it feels like there is a turning point where Patrick Stewart starts taking the role seriously (Symbiosis) and understands that he's depicting Starfleet as an institution and the restrictions (the prime directive) that institutions must enforce. The fellow actors also seem to take the series more seriously after that episode while before it felt campy.

It's clear he brought a gravitas to the role that inspired a team of young/new actors around him. As he later joked in Making it So, he once took things too seriously on set yelling "We're not here to have fun!" It became more than a job/role to him and in doing so he and the other actors inspired a science-inclined subset of a generation and brought Roddenberry's utopia vision to life.

Patrick Stewart was no slouch in using what power he had to protect, either: https://www.youtube.com/watch?v=TqFaiVNuy1k

...who who once said "The strength of a civilization is not measured by its ability to fight wars, but rather by its ability to prevent them" which I find very Trecklike. Live long and Prosper.

I disagree. Not only did he continually & substantially contribute to Picard's character, it clearly aligns with his own values and ethics as demonstrated over and over in interviews. Why not both? That seems like the obviously best perspective.

This very much. Post-Roddenberry Trek mostly misses the point.