That is indeed annoying with thrips, you can regularly spray for a whole period so that they are under control, but if you do not think about it for a few weeks, they are back again ....
Maybe you are right when you say that it does not help to treat new plants, there are too many host plants in the garden and beyond from which the infestation keeps returning.
I know some daylily collectors here in Belgium who have already determined rust on certain cultivars but for the time being it remains under control, we live here in a zone 8 but the winters can lead to severe frost periods some years, min 10 ° to min 15 ° C occurs regularly here.
