IMO, Spectra is by far your best bet. Early Hobies used a heavy monofilament that was stiff, sometimes binding in the rudder tubes and frequently kinking -- felt and operated very much like wire. The steering response was quite stiff. Spectra was a huge improvement!
Your Spectra lines have obviously served you well for several years now. It is extremely strong, flexible, and resistant to the elements. About the only thing that causes it to fail is a sharp or ragged edge. I don't think you need to replace it unless you observe some damage or wear upon careful inspection. If you do replace it, I'd recommend the same thing.
BTW, it's always good to know how to steer with a paddle. It's easy to do with a little practice. You never know when you might break a rudder pin with those high speed electric motors!
PS: IMO, two-30 lb thrust motors won't give you much more speed with the Outback than one. Doubling the thrust output on the Torqeedo resulted in about 0.1 additional MPH from 5.3 to 5.4. That's about all the boat will give you due to hull speed. Four to 4.5 MPH gives you a good combination of range and speed and 30 lb. thrust ought to just about do the job. You're much better off investing in a spare battery.
