I’m fairness, I do a lot of DIY but I don’t do doors and I don’t do drywall. Drywall is easy to patch but difficult to look perfect. Drywall in my laundry room? Sure. Living room? Handyman

If you up the size, consider 2x12's instead of 2x10's. Also, laminating them together is easy but lifting the entire post into the hole is challenging - definitely something to think through. If you're close to home depot then I'd rent a lift. If you want a platform, I bought a 6' wide wooden spool on craigslist for $50.

I copied what this guy did for the cable issue: https://www.youtube.com/watch?v=tZVidnXewBQ

Also check this video out: https://www.youtube.com/watch?v=a1KPl-M-dpM&t=0s

225' is a massive amount of tension.

I did a zip-line with no trees and had the same issue. Wood that size is hard to find and extremely expensive. Ended up buying four 2"x10"x16' and laminated them together with glue/clamps/nails. Then put them in a 6 foot hole filled with concrete. It's very important that the thin edge of each board is pointing towards the zip-line.

If you don't need the elevation, I'd keep the top post as low as possible to reduce the torque.

There are online calculators that will calculate the tension of your cable. Then another "beam deflection" calculator which will tell you how much your wood will bend under that load.

Also - I use services like justanswer.com to double check my work. It's a fast/cheap way to chat with an engineer.