See the two commits:
Revert "Revert "Start processes with kstart5 if available"" This reverts commit 4baa4cba. We can fix the issue that prompted the reversion in this repo; see next commit.
Fix launching commands with args using kstart5 By prepending "--" to the command, kstart5 parses it all as one thing, so additional arguments are executed as expected. BUG: 433362 FIXED-IN: 5.86
Second commit uses the authorship of the fellow who discovered this fix in https://bugs.kde.org/show_bug.cgi?id=433362#c11 :)