I had thought about lowering and raising too. Mine is fixed since it is only 9 foot (3 metres) high. I figure I can get to the SS lines easily with a ladder and lower the hay bailing twine that I am going to attach.
If you went above 3m I think raising & lowering would be a definite advantage.
There are plenty of trellis designs that can be raised or lowered and factor in things like raising/lowering just one plant or a whole row of them.
If you are thinking of a 'clothes line' design like mine then maybe the frame could be a bit of box steel so that it is not as heavy as a 3-5 metre length of hardwood timber? I just used the hardwood because I got it cheap. Then you could have the uprights hinging at their base (if not secured to a pot like mine) and use the side tensioning cables to keep them upright with maybe a winch of some sort on them?
A better idea is:
I have also seen where they have a rigid clothes line T frame like mine but the horizontal cables go through the horizontal frame arms, then come down diagonally and are all joined to the side tensioning cable. The side tensioning cable is thus used to tension the top horizontal cables not hold the frame up. I did not do it this way because the cables also secure my frame in place. If you had the uprights dug into or concreted into the ground then they could support themselves and you could run the cables through them as described above.
The other thing I could do with mine is to tie bailing twine to the fence, then over the top wires and down to the pot. Undo at fence to lower hops, I wouldn't need to go up a ladder then. Easy enough to zip tie something on the ss cable for the twine to pass through to stop sideways movement.
I hope my rambling helps you decide on your trellis design.