I am using old, weaker scooters.. Dacor.
If you are diving in 100 feet of water with 150 ft of line, you are doing it wrong. You probably want around 300 feet of line out for those conditions, unless there is no current and/or you are not scootering anywhere fast.
The whole reason I use a line stiffener is because I screw up and get the floatline wrapped on my tank valve, Then when I stop, I have an uncontrolled float pulling me through the water (from the back of my neck) - how someone would view this as a desirable situation is a mystery to me. Usually I am alone and have to take the entire tank off just to free myself from the entanglement.
As for swell causing you problems, some of the solutions are: more line, or using an elastic section for the last few feet of the line where the float attaches and/or using 2 or more smaller floats in series,
A 3-4 foot bungi section between smaller floats works really well to insulate you from rough surface conditions. My buddy use to have that and it was nice, but I never bothered to make that sort of configuration, probably because we use much more scope than you seem to do.