Layout, colour and a single clear button will do more for you than any clever subject line.
Getting this right is less about clever tooling and more about consistency. Pick a rule, apply it to every customer, and let the results accumulate over a full year before you judge it.
What to try first
Start with a single segment and a single offer. Once the numbers are steady, add a second variant and compare them honestly over at least a few hundred sends.
The programmes that work are almost always the boring ones that simply ran for long enough to matter.