One way or another you will need to build a landing platform. And stairs. So, you can use the back two 6x6s as both a platform base and an anchor for the guy wires plus, possibly, flood lights.

FYI, that's too much of a drop. The max. slope for a zipline is 3' of vertical per 100' of horizontal with no brake, and 6' of vertical if you are using a proper brake:
https://www.ziplinegear.com/pages/how-to-build-a-zip-line-section-one-concepts#:~:text=ZIP%20LINE%20MUST%20NEVER%20EXCEED,MAY%20CAUSE%20INJURY%20OR%20DEATH.

Please do not make a death trap zipline that drops 25' over only 225 feet of vertical.

You don't need to transport the pole intact, and rental trucks can be really cheap for a few hours. You would just need to find one and cut it into more manageable lengths for the truck.

Since they're usually 30-60 feet, even on the low end you could easily get two posts out of it. For example, this one is 35 feet and about 640 pounds. Even cutting it in half is manageable for you and two friends (ideally 3 and maybe even some pulleys or other assistance).

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You might even be able to find somewhere nearby that you can buy them from, but for that you really might need to talk to a utility contractor, and at that point it's probably cheaper to ask if you can get one they're going to replace so they don't have to haul it away themselves.

So would you drill a hole at say 7ft above the ground on the 6x6, run the wire through it and then anchor it a block of concrete on the ground? If it comes to that I guess I can go that direction but the presence of a wire going from the post to the ground with kids running around creates a possible injury risk.

How about just use a 6x6 and add guy wires behind it? The 6x6 is more than enough for the vertical load, it’s the crazy cable tension that requires such a stout pole.

Can't drag it on the road that way :(

Zero trees in the vicinity of the start area. I do have a playset but there are warnings about not connecting it to playsets as they are not usually sturdy enough. All the vertical posts on that are 4x4s and I'd bet they'd snap under the load. I don't really want to build a big structure, just looking to try and comply with the load requirements with a vertical substitute for the 12" diameter post.

Usually people just drag old utility poles with a chain. Had a friend do that when he built his pole barn.

Could you mount the high side to the tree instead? What about building a platform instead of a single pole?

Not that I'm aware of. I also do not have a vehicle or flatbed big enough to transport something so large so getting someone to have, let alone deliver, said post is not going anywhere. I'll look into the steel post idea.

Are there any telephone poles being replaced near you? Utility contractors can help find those.

You might be able to use a smaller diameter with steel post.

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Nice work. The lights make it Uber cool

I redid my pantry a couple years ago with wood shelves. I used 3/4 ply with a poplar face for extra support. Here are a few pics

https://imgur.com/gallery/IcHMZvt

That will be more than strong enough. We're using these pine 1x4-based shelves from Ikea in our pantry. Five shelves per unit with many of them completely covered in canned goods. There has been so sag in 10 years.

Mine were 11.5" deep x 5 feet long. I had some short ones also but that was the length of the majority of them.

What you’ve described is almost exactly what I did in my bus with wood glue and pocket screws. I used dimensional 2x2 as it’s stronger than the framing stuff. It has held up over more than 6,000 miles now. The 3/4 ply on top is overkill in my opinion. Use a good quality ACX and you’ll be fine with .5”. I used .5” on top and bottom of the 2x2 frame. There’s been no sag and I’d guess it’s holding 60-70 lbs plus the forces of driving.

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I wish I was more artistic but I can't even draw stick figures... I'll probably go with your cabinet-grade veneer recommendation. :)

Sagulator only calculates the sag of the wood shelf, not the frame, correct? I am a mechanical engineer (a computational modeler) and I tried to do the sag calculations myself with the frame but wood properties vary a lot and depending on what I chose, I got different results. Hence, maybe someone here has practical experience which supersedes my calculations.

How wide were your shelves? I think the shelves in the link look somewhat comparable (based on visuals) but maybe a little smaller than what we have.

Well, yeah, you can stain MDF, but it usually doesn't work well since there's no grain. Most people will do a faux grain on the MDF, but a nice plywood with a cabinet grade veneer is probably easier, if not exactly cheaper.

Sag calculator: https://woodbin.com/calcs/sagulator/

I built mine similar to how this guy did his. They are strong enough for me (220lbs) to lay down on no problem.

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https://www.pinterest.com/pin/36380709480609166/?mt=login

I am told that I have to stain :). My better half wants the wood look. I am assuming that would exclude MDF? I have never worked with MDF before.

I saw some videos where people build these frames in 2x3 and sat on them (basically weight of a person is not an issue -- seemingly). That's why I was thinking about 2x2.

I'm sorry, I forgot to mention the depth... 11-3/4" or 12" total space.