Hi there and kuddus for planning.
Everything in life is a collection of trade-offs and the ones you make depend on your personal goals and your personality. Philosophy aside, I am setting up a 22 long, same as your tank but a foot shorter. I love the "long" tanks but they have their own set of challanges.
- Substrate: I would not do layers. Over time, they will get mixed and the look would drive me crazy. Regardless of the substrate you chose, you will still have to fertilize the tank sooner then later. Pick the one you like the looks of, know what it is, deal with it accordingly.
- Depth: for that tank and most plants, ~ 2" is a good compromise
- co2: this is where the tank's "odd" dimentions make it interesting: it is long and shallow and getting the circulation vs flow gets tricky. Say, you are using a single filter. Your goal is to have a gentle current from one end of the tank to the other, without the substrate and plants and fish being blown all over the place. That goal affects your scaping choices: placing large solid driftwood / large stones smack in the middle will affect that current.
I am not giving you a specific answer to the above because there are many different ways to deal with it. Be happy to come back to that subject and throw some ideas back and forth.
Above all, have fun and enjoy.