@Johannes Schneider, This issue has been reported in CONFSERVER-24804 which is marked as gathering interest. I also found a similar Cloud issue that was marked as Won't fix which is unfortunate but somewhat understandable I guess.
The issue is that Safari's shortcuts take precedence. Now if Confluence was a client side app then you could remap but not the case when browser based AFAIK.
Workarounds:
use a different browser, e.g. Firefox works
use the insert media icon vs. shortcut keys
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