I don’t think you can reload only the panel, but reloading plasmashell should be painless. There are a few ways:
systemctl restart --user plasma-plasmashell
plasmashell --replace
kquitapp6 plasmashell
(&& plasmashell
if it doesn’t restart automatically)killall plasmashell
(&& plasmashell
if it doesn’t restart automatically)
I prefer using the systemctl method since it deals directly with the service.