Well I’m a complete Noob here and started off with a similar setup.
I’m not going for an MQTT gateway, just Ethernet but my radio problems were similar with similar debug output.
I have an Etherten board and Rfm69 radio.
I’ve now got it working great. I didn’t change and code in w5100.h. What I did do was move the NSS from pin 10 down to pin 6 (7-9 was for the status LEDs)
So the only changes I have are as follows..

#define MY_RADIO_RFM69
#define MY_RFM69_NEW_DRIVER
#define MY_RF69_SPI_CS 6
#define MY_RF69_IRQ_PIN 2
#define MY_RFM69_FREQUENCY (RFM69_433MHZ)

I thought I’d go with the new driver from the outset. I don’t think I needed to define the IRQ pin but I did it anyway. Moving CS was the fix for me initially to get past the radio errors. Then the final key was defining the frequency. Before I did that everything looked good but I couldn’t get a node talking to the gateway. As soon as I defined the frequency it all started working.
Now I just need to grow my hair back.