How I finally solved my W5500 nightmare - check the power pins first

Mine turned out to be the reset pin floating because I'd assumed the chip whould come up fine on its own. It would sort of start, then hang halfway and never answer on SPI. Tied that pin to my microcontroller GPIO with a proper delay before touching anything else and it suddenly behaved. Two evenings lost staring at logic traces before I checked the pin list properly, which is exactly the habit you're describing.

The other thing that got me once on a similar board was the 3.3V rail sagging the moment I plugged a cable in. Measured fine at idle, then dropped low enought under load that the chip would drop off the network. I only found it because I left my meter connected while plugging and unplugging the cable out of frustration. A bigger capacitor near the chip sorted it.

So I'd add to your checklist: watch the supply while the thing is actually working, not just on the bench with nothing connected. And don't trust that a pin looks connected because the solder shines nicely. Half my worst bugs were things I could only see with the meter, never with my eyes.

Same lesson here but on the clock side. Mine passed every supply check and still dropped packets randomly. Turned out I was running the SPI bus at 33 MHz over about 10 cm of jumper wires on a breadboard. Dropped it to 8 MHz and it's been rock solid since. If you're prototyping on wires rather than a proper PCB, halve whatever clock speed you think it can handle and work up from there. Half the time the chip gets blamed for what the wiring is doing.

Funny how these things go, mine ended up being half hardware half software. The chip was fine, but I'd set up my SPI mode wrong and it looked exactly like a dead board. What saved me was reading the version register over SPI before touching any network code. It always returns the same fixed value, so if that comes back garbage you know the bus is the problem, not your stack. Took me way too long to figure out I coud test the chip in isolation like that. Now it's the first thing I do on any new board.