I spent some time configuring my RPi5B as a backup router and did the live test with LAN/WAN connected not double NATed as above. Results where similar with the bufferbloat speed test with or without SQM enabled. The RPi5B is just not powerful enough to run snort at gigabit speeds.
With SQM enabled:
Without SQM enabled:
My connection maxes out around 950 Mbps downstream without snort running on the RPi5B.