(disclaimer: I don’t quite match the criteria in your last sentence but I choose to reply anyway 
)
It sounds like you are at a point where you would really benefit from using one of the calculators out there that model tool deflection (which is going to be the predominant constraint when using such long and thin endmills).
Check out @gmack’s calculator here:
You can estimate deflection, and then decide whether the deflection value is acceptable (which depends on the accuracy you are after).
It also estimates cutting forces.
Me, I would say:
- max out RPM to minimize cutting forces
- adjust feedrate to get a “low but high enough chipload” (which depends on the tool size and geometry). I would not go much below 0.0005".
- but above all, dial down the depth per pass, which in my opinion will be the key to no break those endmills.