Status: after two days of intensive calculation the whopping 1MB CalDAV import failed somewhere between 87-100% and I have no clue what was done and what needs to be fixed. #Thunderbird

Fortunately I found a solution that did the job in 5 mins at server-side:

https://www.reddit.com/r/selfhosted/comments/jbnu1l/how_would_i_push_an_ics_to_a_caldav_server/