Congratulations on the build.
Keep in mind it takes great skill to weld aluminium as thin as this without distortion, especially 3mm. One comment often made is the extra fairing compound and paint
needed can result in a finished structure that is no lighter than starting with slightly thicker aluminium material. The stiffness rises roughly as the cube of the thickness so small differences In thickness make a appreciable difference to the final structure.
6061 has significantly lower corrosion
resistance than any of the five series aluminiums so try and limit its use as much as possible. These days 6061 can be replaced with computer cut 5383 or 5083 for many applications. Unfortunately, many older designs use T shaped stringers and the only practical way of producing these shapes is via an extrusion. This is not possible with the five series. Nevertheless, all flat bar and T shaped bulkheads can be cut from 5 series, reducing the corrosion potential.
below is an example. All the aluminium shown including the stringers is 5 series.
6061 is also an older aluminium series that has largely been replaced in more critical situations with newer versions of the six series such as 6063, 6082 etc. These are typically more costly but have more suitable properties such as higher corrosion resistance (although nowhere near the 5 series) and strength.
The take home message is do plenty of research
before ordering materials.