Ali, your riding is great but you are pretty stupid at engineering.
So listen here, boy. There are no such thing as overkill in terms of reliability in street-trials(you are a smart boy so tell us why by yourself), where weight doesn't matter and 12 kg is absolutely fine.
20mm axle and well-designed fork will weight same(or slightly more (no more than 50 gramms more if we compare both fork and axle together)) as 15mm fork with axle and would be stronger at the same time.
About RnD, they work are not so hard. Choose proper thikness of tubes and proper welding places. Construction of street-trials and BMX frames are very strong by itself. So even more than RND there are quality of production (welding, thermal threating) matters.
And this is fekking steel, it's far more easier to construct and produce than an AL frame. And also it's cheaper to design, cheaper to produce and material by itself is cheaper.