How to run a command to before starting systemctl service

As systemctl run as root you can use sudo to switch to another user without password prompt. Example:

root@leo-pc:~# sudo -u leonid bash
leonid@leo-pc:~$ 

EDIT: after understanding the actual issue

Based on the OpenVINO documentation: Install Intel® Distribution of OpenVINO™ toolkit for Linux.

Create a script which contains both commands:

#!/bin/bash
## /yourscript.sh
source /opt/intel/openvino/bin/setupvars.sh
/usr/bin/python3 /home/john/Documents/ThingTraxVisionPython

Then call this script from your service file:

[Unit]
Description=Launch application

[Service]
User=john
WorkingDirectory=/home/thingtrax/Documents/ThingTraxVisionPython
Environment=DISPLAY=:0
ExecStart=/yourscript.sh
Restart=on-failure
RestartSec=30s

[Install]
WantedBy=graphical.target

First version

I'm not shure to completely understand your issue, but if you need the openvino service to be run as root and the ExecStartPre only to run as John, you can probably create a shell script that logs in as John and launched from ExecStartPre:

#!/bin/bash
## /yourscript.sh
su -l john -c '/opt/intel/openvino/bin/setupvars.sh' -

Note: don't forget the last dash at the end which will set the John environment variables.

Then add execution rights to it:

# chmod +x /yourscript.sh

End to finish, here is the service file afterwards:

[Unit]
Description=Launch application

[Service]
## removed the user param
WorkingDirectory=/home/thingtrax/Documents/ThingTraxVisionPython
Environment=DISPLAY=:0
ExecStartPre=/yourscript.sh ## changed the script to yourscript.sh
ExecStart=/usr/bin/python3 /home/john/Documents/ThingTraxVisionPython/rtsp_ttfr.py
Restart=on-failure
RestartSec=30s

[Install]
WantedBy=graphical.target