I’m probably the least experienced CoH tanker answering your question, but I have only 1 real toon and she’s a rad/dark tanker and a gas. Septic Tank is decidedly off meta however, I went psi epic rather than soul which most builds recommend.
I think the synergy between rad armor and dark melee works fairly well. Grass is always greener but I think the synergies on SS are probably better.
Problem as I see it is that Rad/Dark wants recharge to maintain durability. Rapid fire of radiation therapy and siphon life for healing and particle barrier for the damage shield is essential for Septic to get out of most of the dumb stuff I get her into. Without defense slot skills for LoTG or knockback powers for the FF procs global recharge relies on 5 slot purple I/o. 5 slots are tough because you need slow resistance very much and run out of space.
I work around this with CJ and weave to let me slot 2 LoTG and psi epic for dominate, which lets me slot 3 slot entomb with the absorb proc and psi tornado with a FF proc. With the gauntlet fist proc on siphon life septic relies on pumping the damage shield rather than avoiding getting hit. I’m plying with the idea of cross punch for another Ff proc.
Im going to disagree slightly with earlier posters and say I’d skip ToF. It’s fun, but you have a bunch of other PB AoE powers in soul drain, shadow maul, radiation therapy and ground zero. Dark consumption is also skippable if you slot radiation therapy with the endurance proc( name escapes me)