If you're using synthetic lifelines you should as mentioned not over tension them. Stretch is limited and very likely the result of the lashings in my experience.
For chafe protection you can buy grommets from a lamp supply store that slot into the holes in stanchions from both ends of the hole. They work great.
That said, I'm not a fan of synthetic lifelines. In my opinion the only valid reason to use them is weight savings.
I've spent a fair amount of time on ocean race
boats, a good number of which have amsteel lines, and the downside is that, because it has low stiffness, grabbing ahold of it, no matter how well it is tensioned, is akin to gripping a wet noodle. Granted, you should not use lifelines as handholds but I think everyone who's spent time at sea knows that you inevitably do, on occasion, and usually those occasions are precisely when you want them to offer steadying support. Synthetic lifelines do not offer that at all
The value equation does not favor synthetic lines either, I don't think, although I'm not sure. Uncoated stainless steel
twisted wire will not cut or chafe or suffer UV degradation. I do know that most racers replace their synthetic lifelines every 5 years or so.
If you're not racing
, it makes sense to me to go with uncoated twisted wire, 1/4" for the top (for better stiffness and hand) and 1/8" for the lowers.
If you shop around, new SS lifelines for a 40' mono should run around $800 including new turnbuckles and gate hooks. There are outfits that you send your old lines to and within a week you'll have your new ones back.