I bought some corrugated metal beds and I've liked them for the couple of years I've had them.
The biggest lesson learned is that 4ft is too far to reach comfortably. I put a trellis arch between two of them and they are too deep to really reach the middle.
Mine are 3 ft deep. I filled them with logs and sticks and then added triple mix. A few years in they are pretty compacted, I will need to get a lot more compost in them. I am experimenting with adding straw for bulk.
I hope this isn't unwelcome advice, but this is a vegan gardening community:
I practice veganic gardening (as much as is possible - I'm sure my triple mix had some animal manure in it) so I don't have direct experience with using manure in my beds. I do want to flag that mixing top soil with manure has a few potential downsides:
If you google raised bed soil recipe or something like that, you will find ideal ratios for different components that will allow good draining and aeration.