joel moskowitz
The mortise in question is pretty long and pretty deep. the one in the picture several inches in one case. the OP is using home made chisels where the geometry isn't correct. Real mortise chisels might be 3/4" in thickness at the bolster but are only 1/4" or 3/8" thick at the cutting edge which tapers in the primary bevel at 20 degrees or so until the secondary bevel so that they aren't thick and cumbersome at all at the tip. A 1" long mortise 1" deep is a snap to chop so is a 1/2" x 1/2 or 3/4". A deep 1/8" mortise is also pretty easy.

A lot of the problem here is that in pursuit of ultra long lasting cutting edges the OP sacrificed the rest of the geometry that makes the chisel work.

We are the only seller of traditional English Mortise chisels in the US. I have always thought in the days when that was all that was there a poor compromise. I put together my set of mortise chisels during trips to the UK in the early 90's.