Back when cables used to wear out there used to be a thingy called a "split ferrule", which took the excess slack out of cables.

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It was a split collar with two diameters (small one fits in the hole the cable goes into on the adjuster, large on fits round the end of the cable) . They used to take up about 6mm or ¼" of cable slack. I think you can still get them at off road bike shops. Or you could bodge one. :???:
It is not common nowadays for cable to develop excess slack - it used to happen when a nipple was sliding off as the solder failed! :???:
IMHO I would try to work out why this is happening. .Take the tank off & look everywhere to see why the cable outer is no longer long enough or the cable inner is stretching!!
Have fun & good luck Jon