You kind of pointed out the problem. No matter how nice they may be those Echos have short exhaust durations, which has predictable impacts on power output.
Maybe try just a muffler mod and a loop of really sharp full chisel and see how the 500 does then? Or try a Narrow Kerf 0.325" bar and chain.
Or do a muffler mod on the 421 (gut the cat), and put one of the GB 68DL lo pro bars on it. It's a full inch longer than that 62DL 18" bars, and an inch shorter than the 20" bar on the 500, so it's almost as long as you wanted. I run one on a heavily ported 42cc Poulan and it pulls it well with a loop of 91VXL. Then you can ditch the Echos and keep and eye open for something larger with some power.
https://leftcoastsupplies.com/product/18-gb-arbor-tech-bar/