Awwww skoda you're doing so well. Please remember these are HUGE jobs you're undertaking - the kind of thing that would take a couple of labourers who have done it dozens of times before hours on end.
I think it's really stressful if you try to rush these things. I know there's this urgent desire to get things looking better, to have a space that is lovely, but I would say that if you are doing the work yourself around other commitments, then you need to give yourself the gift of the time needed to do the job! It took me an entire summer - from March to September - to get my back garden into a state where I could even begin to start planting. (I had a lot of concrete to remove, and I did it by hand). It looked like a dog's dinner for months, but it was worth it.
With the lawn edging- think about maintenance. You need to be able to get a mower over it, right? So the very last thing you want is any kind of edging that comes up above the surface of the turf. Your mower will hit it, or you won't be able to get right to the edge and will have to do that bit painstakingly with shears, which will make you swear. A lot. If you love sleepers, then make sure they're treated and sink them into the ground. However, you may be better off with a product like Everedge.
For planting - if you give yourself more time, you're also giving yourself greater opportunities to find things cheap. Aldi, Lidl, Morrisons do a LOT of the kind of things you are looking for, for a fraction of the price of a garden centre. But they're not all in at once, you have to hunt them down. Furthermore, with time you can look in the search & rescue sections of garden centres and find things that work with your scheme - I think most of us on the forum have things we've found this way!!
You can also propagate easier stuff from seed so that you have lots of plants for a couple of quid.
The other thing is that it's a mistake to think a garden is ever 'finished'. Things outgrow spaces, or they come to the end of shorter lives, or get diseased, or you find they are in the wrong place and need more/less light, water, etc. Your mini-ecosystem will be constantly evolving. The good news about this is that, if you make a mistake with the organic stuff, it can usually be undone fairly cheaply and easily.
Where it gets more expensive is where you're dealing with hard landscaping materials, which is why it's a good idea to take some time over that stuff until you are happy. 