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Turn tracks are excellent. They build a nice turn track at ClubMX. Seems to change a lot, but they include a few small jumps. Probably see them on their videos. 400x220 is bigger than a football field, maybe close to the area used by SX tracks in football stadiums? Look at the track maps for layout ideas, then add jumps or whatever to meet your skills.

I would highly recommend riding whatever layout you come up with first before moving any dirt so you can confirm that it’s going to flow nicely together. I’ve built some sections before that I thought looked great but when I actually rode them, it was a different story. My other piece of advice is to keep in mind any low spots for rain water runoff. I know you said it’s relatively flat but if there are any dips at all, try to layout your lanes so the water will run off them sideways, not pool right in the middle of...

This tip is probably obvious to most but when I was younger I learned the hard way not to ever take dirt from the riding lane to build anything. Always grab dirt from the side of the lane…unless you want a Koi pond before the face of your jump.

You don't need jumps. I have way more fun on Washougals "grass" turn track than I do on the main track. It's far less risky and it makes me a better rider. Any idiot can jump. Corner speed is what makes the fast guys fast.

I would start out by riding the track in so that you can find something that flows. Set up some markers to help you get it ridden in at first and keep in mind how wide you want it and set an inside/outside line. Once that is done, then start to add obstacles.

Always let your buddies try it out first.

I would do doubles with forgiving landings before tables. I know you said you can source dirt, but tables take up a ton of dirt and time, especially if you aren’t borrowing from hills or bottoms incorporated into the design. Just my two cents after moving way too much dirt in my life with way too small of equipment lol