It kinda looks fungal to me but hard to tell with out a close up of the needles. We had a very humid year last year so there could be alot of fungal issues this year.
Many fungi infect needles one year but you dont see the effects of the infection until the next year.
You said you applied copper fungicide every 3 weeks.
When did you start appling it and when did you stop?
If its not fungal then its got to be a root/watering issue.
I also have scots pines and use a 1:1:1 akadama, pumice and lava and I dont see this on my trees. Mine get watered every day during the hottest months (Mid June-Mid September) then every other day in May-June; Sept-Nov/Dec. I will shut off the water system if we are slated to get 0.5 inches of rain or more. I have never seen an overwatering issue with that mix and that watering regimen. I dont use pond baskets but I do use a plastic basic lined with embroidery screen from a craft store that is the same sort of depth and I dont see that staying that wet.