An old fake tree could totally work, as long as you have something that could cut it into smaller pieces.
I'm sure they make 'em big. Here's some I found, they're expensive though! You could probably make something.
The trick is that it should be pretty rigid, so that's the hard part.
My standard wreath is a bit smaller than yours, maybe 3 feet diameter, with a base of blue spruce. (Usually what I cut off the bottom of my Christmas tree, though sometimes I buy a plain blue spruce wreath and then fancy it up.) Trader Joe's makes door swags that are cheap, I buy those and disassemble for parts and add that -- usually holly, juniper, white pine, boxwood, etc. Usually I add some ornaments too (glass orbs, I have a variety of gold-copper ones, about 6 different tones/colors [all gold or copper but different finishes and shades] , that are all a little weathered and cool looking. I add wire to the hanger part and then attach the wire to the wreath frame).
I think I remember you have some oversized ornaments, those could look great on a huge wreath.
I also add a big bow, colors vary.