If I disable CSM, the iGPU gets disabled (I don't understand why they are linked) and I still have the recurring log entries. Above 4G Decoding was already enabled but I did have Resizable BAR support set to Disabled. I enabled that and the system was stable for about 90 minutes, then the log spam started again. I tried the workaround of editing my Go file so it looks like below and then rebooting but that also did not work.
#!/bin/bash
# Start the Management Utility
/usr/local/sbin/emhttp &
modprobe i915
# Suppress ACPI Error messages
echo ":msg,contains,\"ACPI Error: AE_ALREADY_EXISTS\" stop" >> /etc/rsyslog.d/01-blocklist.conf
echo ":msg,contains,\"ACPI Error: Aborting method\" stop" >> /etc/rsyslog.d/01-blocklist.conf
echo ":msg,contains,\"ACPI BIOS Error (bug)\" stop" >> /etc/rsyslog.d/01-blocklist.conf
/etc/rc.d/rc.rsyslogd restart