I'd agree with that except the "better sfx company" part. TOS had, in total, six of the best visual effects companies in Hollywood working on the show, a far greater assemblage of VFX talent and innovation than any previous or contemporary television series. By the standards of the day, it was the most visually spectacular and groundbreaking TV series ever made. The talent or skill of the artists was never an issue, only the available time and budget, both of which a feature film would naturally have had in far greater abundance.
But you're right that the FX only represented the best approximation of the intent that they were able to manage, and they would've gladly done better given the time and resources. It's best to think of these things as artistic representations rather than literal depictions. Have a dozen artists paint the same model and every painting will look different. Different FX team's interpretations of the Enterprise, or different makeup artists' interpretations of Klingons or Andorians, are like different comic book artists' character designs for Clark Kent or Peter Parker. The character may look very different in different issues, but you accept that it's the same person and the difference is due to the artists expressing their individual styles.