I agree with everything you said except the foam making great bass traps. I dosagree. Even reallt thick foam has poor bass trapping qualities.
If you buy mineral wool (which is SOOO frigging itchy its not even funny, wear gloves and long sleeves) in 2 foot by 4 foot panels in the 2 inch thickness, there are a few simple ways to do it quick, easy and cheap.
They 2 things I did....
1st, I ran up to Home Depot after I got my 2 bundles of mineral wool, and had them cut me a bunch of 1x4's into 2 foot and 4 foot pieces and picked up up some small nails. Ran it all home, nailed together some 2 foot by 4 foot frames, thumbtacked some burlap (from Walmarts fabric section, cheap as hell) to the back of it stretched kind of tight. Loaded 2 panels of mineral wool into them (4 inches thick for bass traps) and then covered the front with burlap and wrapped it around to the back and staple and/or thumbtack it. They look very nice and hang right on the wall like a picture or stand in the corner. Oh, for corner traps (which you always put diagonally so there is an air space behind them) I used 6 by 2 panels just to cover more height (had a few piece of 6 foot 1x4 cut too)
The other thing I did which was even cheaper, and I use this for reflections off the wall and behind and over my mix station... cut a piece of thick cardbaord 2x4 wide, lay a 2 inch panel of mineral wool on it, cover the mineral wool with burlap and wrap it around. Glue it and let it dry, and then glue a couple of picture hangers on the cardboard. Hang on the wall like a picture. Works so well its almost funny. The bass trap with the 4 inch in it are like blackholes when it comes to sound. Scream into one with your nose almost touching it and its like a vacuum. Nothing comes back at all. Good stuff.
For bass though, you want your panels at least 4 inches thick.
FOr treble stuff, foam will be fine, but mineral wool or rigid fiberglass is still better there too.