I certainly value mobility, at least from a tactical standpoint, I just put more weight on the Charybdis and Clades having a stable line to dance around; a line that Tartarus cruisers need to provide and that their Siege Mode system seems designed to do. I perhaps don't value mobility as much as I should from the idiot AI doing "stupid things that get it killed" standpoint that you referenced. Since you made the current iteration of it, I imagine you have more experience in how the AI actually handles changes to the decision loop and the unintended consequences that can arise. If the current iteration is the best that can reasonably be done without it becoming a bloated mess, that's fine. Obviously there's not much point devoting a lot of time to it if the system may change, anyway.
That being said, using aggressive captains on Tartarus ships, along with omni shield conversion (lowers arc to trigger Siege Mode much more reliably), Hardened Shields, Shield Modulation, and as much flux capacity as you can stuff in does the job. They'll stay on the front line due to aggressive, and the shield stacking gives them the staying power to hold it while the faster ships move in and out as needed.
I imagine you knew all that, I'm just posting it as a tip for the next person that comes in and searches for Siege Mode.