There is some scientific evidence on the effectiveness of penile traction therapy, which is done with a penis extender. Reports claim gains of around 2 cm (0.79”) in six months, using the device 4-6 hours per day. That is for the flaccid penis; gains in erect length tend to be a little lower. Penis extenders produce results similar to surgery without affecting angle in erection and improving erectile function. They are also quite affordable.
If I were you, I would try a penis extender for 6 months and see how it goes. Provided that after 6 months you did not progress, you should revise your method, specially extending hours. Tension is not as important as time.
If you did not gain anything in the past, you either tried an ineffective method or you did it wrongly. Personally, I do not jelq or make any kind of manual stretch. I stick with the extender, which is the only method that has at least some scientific evidence behind. Jelqs and stretches could help, but I won’t spend time and effort doing something which is not proven apart from anecdotal “evidence”, whose gains -I suspect- are probably temporary and not permanent.