There are nematodes for leather jackets. We get loads on the allotment and one of my friends used the nematodes with a lot of success.
Might try that. My lawn isn’t on the best soil (was new build 5 years ago) but has been ok, but this year it’s proper buggered. My neighbour has a lawn care service and I asked him to have a look but he said it was knackered. He recommended returfing properly (he didn’t do it so wasn’t try to get business) but it looks a pain & the professional quotes I got said to watch until next March now. Bit sad as although only small was a decent back garden