the structure needed to make a plane that large -- will be your biggest challange.
As far as airfoils - well -- your choice -for what you are shooting for -is fine.
keeping it from twisting or breaking and still being really light is the real task.
On really big models (I have a 127" span 2600 sq in aerobatic turkey) we found that the "surprise" in the whole effort -is the power needed to pull it along - same thing full scale guys found out when the early, really big planes were designed in the 1930's
The wing will lift the weight but the power needed to move it along goes up at an alarming rate.
So pick an airfoil which has the best strength -unfortunately - the thicker the better for this .
We did some big biplanes which had to have 10% foils to make the thing responsive for aerobatics .
the structure was stressed skin-plus top n bottom spruce full span surface spars . the wings flex easily but don't twist readily. the tubes you plan on using to hook the panels together -- really add weight . -once the structure at the joints is all added in.
If you can afford it - one large full span composite I beam spar would be a good possibility-OR a full span carbon fibre tube which has a diameter close to the proposed wing thickness.
If you can get anywhre near 20 lbs - no aids such as flaps will be needed -