if you use wire pull strings there is no stretch - you can use braided fishing wire and there is next to no stretch with them - you may also be able to get kevlar fishing line - we use springs in the tail surfaces and a a pull line - there is no maintenance required.
I've been thinking of designs using pull strings. What do you do to account for the changes in length as a function of stretch or even CTE? I've seen some of the both positive and negative CTE on some of the strings... nylon, monofilament, PE braided line, Dyneema, polyester, etc. Do you re-tie every flight day or have something that will give, like springs?