I am not too sure because I have never done anything similar myself but you could write a bash script like this
#!/bin/bash
sleep 5
killall plasmashell
kstart5 plasmashell
then change it's permissions with
chmod 755 bashScriptName
and then add this bash script to autostart via
System Settings --> Startup and shutdown -->Autostart --> Add --> Add login script
and then choose the bash script.
Though honestly the wait time of 5 seconds seem too long to me. I for sure can't wait that long. Why not go for 1 or 0.5 instead??