Is $27,000 to disassemble and replace retaining walls with wood timbers a fair price?

My wife and I bought this property 5 years ago knowing these retaining walls would need to be fixed, and now is the time. I was quoted $27k for the work, which I find insane and makes me question how anyone affords home repairs/improvements. After hearing the quote, my wife and I are considering a DIY approach. We plan to disassemble the old walls using our tractor, then slope the area, put down landscape fabric/tarps, and drop in larger rocks (softball to basketball sized). I'm concerned that if we pull down the larger wall, which retains the dirt where the deck posts are, the dirt might collapse and affect the deck's stability. I need advice on the process order and whether the posts are far enough back to avoid this issue. Most of the existing block is quite degraded, crumbly in places, and will likely be powder in 5-10 years. According to the inspector, the current blocks are not the right application for the wall's type, height, and span, and they weren’t anchored properly; we were told it needs to be completely replaced or redone. I find the $27k quote outlandish and borderline offensive, especially since the house itself cost $300k, and I'm not comfortable spending 10% of the house cost on a landscape feature.