This little python script adds two reminders to each entry in an ics calendar and forgets all reminders previously present.
For it to work, you need python3-icalendar and datetime, installable from debian repos or, I am sure from pip repos.
The script should be self-explanatory. In order to use it, you must provide an input path at the top of the script and an output path. Then you should add times to the reminders, also at the top of the script.
I hope, it helps you. You can do almost whatever you want with this, see the (un)license.