I'd just say that if you have not installed or built a retaining wall yourself previously, then this is not a good beginner DIY project. While much of gardening and landscaping is easy to do, some things are best left to professionals or those experienced in that sort of task.

Wood is never a good choice for a retaining wall as any wood in direct contact with soil will rot, rainy climate or not. And railroad ties/sleepers are considered hazardous waste here. I'd be skeptical of anyone offering them for sale. Stick with the wall blocks - that's what they are intended for.

How old are you? Do you have a good back? Is this your forever home? Railroad ties might give you 20 years Use landscape timber and you’ll be thinking about rebuilding in 10 or so; probably sooner in the PW NW. I would suggest using stackable block with a locking “lip” or similar design like you can find at home improvement centers.

Using 8 x 16 inch block, with proper construction, footing, burying the first course, etc., you can go 3 feet high with no problems. Larger block would allow you to go higher.

Installing a French drain is not a project for a beginning gardener!!

No to wood.

A fundamental problem with the existing wall and why it moved and got wiggly is because the blocks are stacked directly one on top of the other with vertical seams along the entire wall. Each course of block needs to be staggered. The block above should sit over the seam/joint/ joining edges of the two blocks below. This gives a wall more stability and resistance to pressure. That means you may have to cut or buy some half blocks for the ends of the wall.

If you need a French drain it should be in the lawn not in the bed between the boulder and block walls. A dry stack stone or block wall drains fine on its own. What you don't want is dirt draining into the wall. So backfill behind the blocks with gravel. The landscape fabric can be used to keep the soil out of the gravel. So it is soil, fabric, gravel, block.

Want to save money? Use the blocks you have. Take down the block wall which in this case is just moving them out of the way. Dig a 6" deep trench as wide as the block and fill it full of gravel to be basically level with the lawn. True level needs to be in the gravel footing. That is your footing. Restack the blocks correctly with each course having the block above covering the seam between the blocks below.

Want it higher? Buy more of the same kind block and add another course or two on top.

gardengal-

We all have to learn some time. When I wanted to install my first French drain, I watched some Youtube videos and did a little reading, and it seemed pretty straightforward to me. If this was a project that was dealing with properly doing drains related to a building's foundations, I would completely agree with you. But since this is just intended to reduce moisture out in the yard, I see it as something that is fairly forgiving.

Im not completely clear on what you want to do . Youre talking about only redoing the row of small bricks in front, and not the big boulders, correct? I hope so - it looks like those rocks are doing what theyre supposed to do.

If only the front tier If so that really is not beyond the means of a noob providing you are good with things like levels and measuring and are artsY-craftsy in other ways and are a stickler for detail. Read up on the topic - Christopher above gave good info, theres more to know about things like sloping the blocks to the rear so rain is more contained behind the wall, etc etc etc. You cant go up much higher cos that would harm the tree, and you woud want to be VERY careful about not chopping up major tree roots or you could lose the tree.

You could hire a garden or landscape designer to come up with a plan for that (and entire yard really ) Some (most?) are quite happy to work with DIY people. A more curving undulating wall would look nice e be more harmonious with the natural shapes of the big rocks. And with a curving wall you could easily go around the tree further out and in such a way to avoid the tree roots.

Id use bigger blocks- more weight wouldnt shift around as much, and visually be more in scale with size of yard and other features.

Those are box store "garden wall" blocks, aka trash. You could either spend a couple weekends messing with them or you could bring in topsoil and just grade the bed down to the lawn and eliminate the wall. I really don't think a wall is needed here and because it's really too small for a real drain tile or drainage area, I think it complicates your stormwater management goals.

As far as what is and is not a DIY project for first timers - it depends a lot on patience. Homeowners who take the time to do research, measure 2-5 times and cut once, and continually check level - they all seem to do just fine. It's the ones who think they're going to bang it out in an hour are the ones who help with job security.