Be not annoyed, oh mighty mods, I post here hoping for more suggestions on Presscilla...

[MERLIncMan]

I watched the video again. It looks like there might be too much friction between the hammer and the column? If so, maybe some roller bearing guides could help?
Also, you might not need a new pump to use the double acting cylinder. It might be possible to just replace the spool valve to one that can toggle between the cylinder ports.

Thank you for the suggestions! The hammer and the column are a little tight, but I can change it as desired; I've got HDPE tape in there with lithium grease, it's nice and slick. The backpressure of the fluid returning through the line is more of an issue than the ways I think.

I've strongly considered the spool in there, but the valve block is the issue rather than the spool - I could bypass the factory valve if I can figure a place for fluid return - then I'd just put in a cheapo 3-way spool valve.