My solution -

Take one end of the line and tie a uni-knot (or any other form of slip-knot) over the other end of the line and then pull the line so that about an inch or so of line protrudes beyond the knot.

If there is a strong pull on the lanyard, it will pull through the uni-knot.

By pulling on the tag end of the knot, you can adjust the tension.

I've used this for years on lanyards I've used for duck calls and dog whistles and it's worked well.