Just reporting that I seem to have solved this.
Earlier I changed my cron.php to run as follows:
sudo snap set nextcloud nextcloud.cron-interval=10m
That however seemed to be the culprit. The cronjobs somehow bugged and would just repeat constantly for some reason. This also explains the numerous cronjob logs with same reqId.
I changed the interval back to default 5 min and after initial cronjobs the writes decreased. Now the cronjob runs every 5 minutes, but the rest of the time there is hardly any writes going on. Everything is now as it should.
