My idea is to investigate with neighbors and find out who may be getting lawn service that may use harmful chemicals to reduce bugs in lawn grass.
You can be a neighborhood embassidor for the native pollinators and butterflies by exchanging plants, holding garden tours and generally educating others about what you are doing.
And as always, keep up the good work!
You can plant some Milkweed, Anaphalis, Antennaria, Hollyhocks, Globe Thistle, Fennel, Dill and Parsley to support various butterflies in the larval stage. Depending on your region, you can plant for more species of Swallowtail. Where are you located?
Butterflies sell. So do bees, but they do very well with their Lepidopteran "wing man".

Rose of Sharon I noticed attracted quite a few. Quince was good too, and hummingbirds loved it too! Even nested in it.

Consider raising honeybees.

It's a nice - though not very cost-effective - hobby;

and then you'll know you're doing something for them.

There's hives now that allow you to tap in to the honey without taking the hive apart..

I am sure it is a fact you have a decline, but in my case I see an increase. Of course i have been in my place only 4 years and i keep increasing the flowers and I don't use pesticide of any kind. So yeah, they have migrated to my yard.

Its the same in the UK, I think that the small help that we gardeners can give them, is no compensation for the damage the pesticides are doing. Most pollinating insects fly long distances, and there only needs to be one treated field in their flight path and they are gone, dead long before they get near your garden.

Sadly I've noticed the same thing down here in Southwest Texas. Not near the bees I used to see in my yard, and I've planted a pollinator garden for them. In the spring when my Wisteria was in bloom I had quite a few, but now I rarely see any.