If it was me, I would be more tempted to just scrape/wire brush all loose paint and rust, then give it 2 coats of a direct to metal paint like hammerite/rustolueam/lidl aldi own brand
The lidl/aldi direct to metal paint is good for the money
I'm not sure if 2 mad collies and a lawn is compatible in a small space?
I would be more tempted to keep the patio which looks in decent shape, get rid of the slate chippings, and plant grass/shrubs/flowers on the banks?
Sharpies are rubbish, they're not permanent.
You can get paint pens (real paint, in a pen!). Think lidl will have some in this weekend, or you can get them any colour you want on ebay. I use them on a lot of hand tools.
Shovels rakes etc I normally mark with linemarking paint (cos it's cheap...
To me it looks nowhere near wide enough or deep enough? Looking on google there's sites saying the concrete should be reinforced, 12" wide, 6" deep. Sounds overkill but that would stand the test of time, unlike what you're got now.