Startup really confuses me. It seems various methods for starting things happen in different locations all over the place. But is there a way to define the order they are loaded?

Specifically... I have an issue where Plex Media Server (PMS) can't be discovered by clients if the server's VPN connection is established prior PMS starting. It was driving me nuts, but I discovered that I could solve the problem by shutting down PMS, shutting down the VPN, turning on PMS and then turning on VPN.

I'm not exactly sure how PMS auto-loads itself at startup, but the VPN is something I defined by editing: /etc/network/if-up.d/slickvpn with the following:

#! /bin/bash

REQUIRED_CONNECTION_NAME="Wired connection 1"
VPN_CONNECTION_NAME="myvpn"

activ_con=$(nmcli con status | grep "${REQUIRED_CONNECTION_NAME}")
activ_vpn=$(nmcli con status | grep "${VPN_CONNECTION_NAME}")
if [ "${activ_con}" -a ! "${activ_vpn}" ];
then
    nmcli con up id "${VPN_CONNECTION_NAME}"
fi

That's a script I found somewhere (I think on this site) when looking for help autoconnecting a VPN.

So I guess /etc/network/if-up.d/ scripts are initiated prior to the PMS service. How can I flip that? Or other solution?