Thank you, Kergarin for the kind words and compliments!
It all started for me when a friend gifted BG to me because I did not want buy it initially... it made my stock propeller designs from before seem like toys
My first attempt in related crafts were a staged Eve lander, which used propellers for pure vertical ascend, and a Kerbin SSTO with rotating ducted propellers for VTOL. I learned a lot from those.
To answer your questions:
There are 16x R25 blades on each EM-64S motor. The motor is set to 60% power and a max RPM of 400. Testing at 16km on Eve showed that this rpm produced the most thrust, for some reason... The whole motor assembly is placed once in the front and back to help with weight distribution and aerodynamic stability. In total, this is 7 tons of dead weight for the electric stage. Sounds like a lot, but in the end its only 12% of the dry mass...
Aerodynamics optimization and TWR become the most important part once you ignite the rocket engines. Ideally you fly prograde most of the time and let the wings lift the craft out of the atmosphere while gaining decent horizontal speed. For me, the best wing incidence was at 3.5 deg. If your initial TWR is too low, then much fuel is wasted just breaking the sound barrier and climbing above 20km where the air gets thinner. Too high TWR might cause yout to accelerate quickly in the thick atmosphere just to throttle down later, or carry to much engine weight.
I have tried a single engine type before, but could not reach enough dV -- diminishing returns due to high dry weight -- without beeing too heavy to fly up to a decent altitude on electric power.
In the end, it is very tricky... It is possible to end up in a "local maxima" of a design without reaching orbit yet, where the only way to make it any better is to make big breaking changes.
Best of luck with your Eve SSTO attempts!
Here is a gallery of my SSTO that shows some more details: https://imgur.com/gallery/afVyAv1
I will upload a craft file later, once I had time to do a complete mission.