So the turtle feels great as an overall, however I can't help but feel like it still needs a dismount strike skill like the other mounts do on their 1 ability. It would make mounting up on the turtle feel a little bit more versatile.
Given the way that the Siege Turtle is designed, it seems to be most effective when used during an open world meta. I think it needs to be just a little bit more versatile. I find that I still prefer using other mounts even during a slow meta "crawl" when escorting the primary NPC/objective, like Yao in the Kaineng Blackout meta. It is more rewarding to round up enemies with my Raptor which allows me to easily dish out burst damage in one combo across 5 targets with my abilities, or to mount my Springer for significant breakbar damage on a high health Champion or Boss target. So it feels like unless I plan to stay on my turtle for an extended period of time (usually with the intention of a friend hopping in), I don't have much incentive to be on it, especially since it's not nearly as agile as the other mounts. My suggestion is to add a completely new skill that dismounts and strikes enemies like other mounts for skill 1, and simply move the current slam skill to skill 2.
The unique functionality for the turtle dismount ability could be that it unloads X number of charges on the jade cannon strikes if there are currently any remaining in the passenger seat, maybe 1 or 2 at maximum since it does deal a significant amount of damage. I feel like there are quite a few possibilities for something unique.
For my idea specifically it would look something like this:
X strike damage for turtle dismount + 1 jade cannon round (if any are available)
With my specific suggestion, this would provide the turtle's dismount skill with more damage than the other mount's skill 1 dismount ability, and moderate CC via 1 jade cannon round, but not outclassing that of the Springer's Cannonball. With the turtle being a heavyweight champion, I personally think it deserves the highest dismount damage. In my mind, this level of versatility should please pretty much any type of player.