Bracket engine, what would you build and why?

Just for starters, I'd never use aluminum rods in a bracket engine despite the many runs. Too many good steel rods that will last a lot longer.

As for inconsistency at the top end, it could be air turbulence, but could also be a lean condition from jetting or fuel delivery. But even slight variances in a breeze can change ET's by a couple of hundredths between runs, which you or the engine have no control of.

Just remember, consistency wins bracket races, not necessarily how fast you are. Concentrate on reliability and ease of maintenance. Fast is fun, but 9.99 has a LOT of additional safety rules that have to be followed too.