Option 6 - An external option
The six-man rotation does keep the door open for another starting pitching acquisition from outside the organization. It's no secret that the Cardinals and Rays were in talks for Tyler Glasnow before he was shipped to Los Angeles, and the Cardinals have also been linked to White Sox ace Dylan Cease. Moreover, if free-agent options such as Jordan Montgomery and Blake Snell have gotten more affordable, it's possible though unlikely that the Cardinals could swoop in on a shorter-term contract for either of the two frontline pitchers.
With starting pitchers as routine-based as they are, it's very possible that Montgomery and Snell will be forced to sign sooner than Boras might want to force the market. Suitors are also growing impatient and many such as the Rangers appear to be tapping out. Since the Cardinals filled their needs as early as they did, they can hold more leverage in such negotiations especially since an extra starter is more of a want than a need.
I believe the Cardinals are still one more top-end starter shy of contending for a World Series championship in 2024. While it looks like they'll roll into the 2024 season without that final piece, it would be wise to get it early and play like a World Series contender for the entire season instead of just the second half. It seems like that was the plan in 2023, but the Cardinals fell apart before they were able to get to the Trade Deadline as contenders.
Jumping on the market early may also help the team avoid paying a hefty price with many other teams looking to add starters at the deadline. After all, Montgomery and Snell can be had for only money right now, and the Cardinals have the rare opportunity to strike so late in the offseason.