Hi @Ryan Halaoui ,
According to your description, there is no available ootb way to create "pop-up" on user's screen in SharePoint. Are you using SharePoint online? You could create a Power Automate flow to send email notification to users as a workaround if you are using SharePoint Online.
Follow the step in Power Automate:
1.Create> Scheduled cloud flow
Note: You could add hour, day, month... in Repeat day.
2.New step > Send an email(V2)
Note: You could select user or group in “To” option.
References:Run flows on a schedule
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