My house sits on what was the bank of a creek bed before they put the highway in a couple blocks over and cut if off. The houses that sit behind me are higher up than I am and their backyards slant straight down. I was shocked to say the least after the first winter came and there was so much water back there you could have put in a skating rink. I got all their run off. There use to be tiny frogs that lived back there and slugs as long as a foot. You'd get up in the morning and go out on the back porch and find slimy slugs on the steps making their way back to where ever they came from. My neighbor, an elderly man who has since passed away, I convinced him we needed to build a berm along the back of the property line. We spent years putting up we'll take your leaves signs in the fall and kept at it until we had quite a berm that held back the water from flowing down into our yards. I still compost all my leaves and other fallen debris back there. I had to do the same thing with the house that sits next to me though we set level next to each other, his backyard was above mine and slanted down toward mine. Which meant I had to get it above his yard first and continue to build above his yard to block flow down into mine. I've been here thirty plus years and it was only maybe four, five years ago a huge tree fell down back there and the logs off it were so big it did the job of finishing off that project pretty much. It actually ended up adding some fun for the grand kids because we stacked them up into a step formation and at the top we took a vinyl top and attached it to trees into a tent like formation so they had a secret hideout back there. Along the back line of the property trees also came in handy once accelerating the formation of the berm. An older lady lived behind me but she stayed in Florida for the most part. Her backyard was full of trees. When she passed away a guy bought the house and was going to use it for a rental. One day he shows up with these fancy logging type clothes, boots and a chainsaw and he has at it taking down the trees. He gets to this one particular tree and it fell across the power line leading to his house. He went to start cutting it at the bottom and we yelled at him he didn't want to do that. We told him you want to cut it as close to the top of the line you can get so what rest above the line the line will go back up and it will fling it off and the other half will drop to the ground. He didn't listen of course, he cut the bottom that put more weight on the line and snapped the line which ended up pulling it out of his electrical box enough to cause the house to catch fire. You could sort of just look at him and see he was out of his grade when it came to being a forester. But we got tons of wood from him and he was more than happy to give it to us. Anyway, I can sympathize with your endeavor having been through myself taking on a lot of water from other properties. It's been a long endeavor but it was well worth it and it was free. That magic word free, not much in this world that's still free.
Sounds like a long-term project for sure! I am glad that we don't get slugs (that I know about) though. The previous owners were "making homes for snails" in the garden, but we have since been able to clear out most of the crap that was housing them. There were heaps, and heaps.
There are a few stories around here of people trying to do their own logging, and it not ending up so great. One had a tree fall on his house. Just because there is plenty of forest, doesn't mean everyone is a lumberjack :)
"Tens of thousands" was the verbal estimate :D
I was thinking after you mentioned retaining wall, cider block may be the cheapest route and it's something you can do yourself to save money. You could do a lot of creative things like plant vining plants out of the top of the blocks, herbs, flowers, put in some bird feeder poles if you are into attracting and feeding birds. Just some food for thought to maybe cut out some of those tens of thousands. It's also something you can say this year this is what part I will accomplish. Good luck whatever you do!