Bird lovers I have questions

[Marthe48]

Should I just put out straight sunflower seeds? I am wondering if there is a bird seed mix that all of the birds will eat and more of each kind in the mix?

I have feeders up and fill them with a birdseed mix, which contains sunflower seeds. I also put out straight sunflower seeds. I have several squirrels who visit my yard and after they eat the sunflower seeds, raid the feeders and dump a lot of seed out of the feeders.
The ground feeding birds eat some of it, but there is so much they leave, not sure if any critters will come along and clean that up. It seems like the sunflower seeds are the most desirable.
I don't fill the feeders every day and I try to hold off putting fresh seed out until the areas under the feeders aren't as seedy. I had been scattering all of the seed on the ground, to slow down the squirrels and chipmunks, but with winter and snow, I turned to the feeders. Also, even with a fence, neighbor cats come in and I want to give the birds as much of a chance as I can.

Thanks for any advice