It’s a Berg 390 so no dog bones to muck about with, the shock is mounted directly to the swing arm so shortening the shock seems the only option save shaving the seat which is a half arsed way of achieving a height réduction
I had FTR Suspension shorten a shock for me ( which they later changed back again!) very good service and quick turn around.
Get it done properly, the spacer inserted internally on the rod will simply limit the amount of travel while shortening the O/A length, but importantly won't affect the majority of the performance, you may or may not need a shorter spring as well.
Some people do the bodge of backing the preload right off and lift the forks up a bit but by comparison it'll handle shite.