Oh yes, good advice you have gotten here.
Compacted red clay is nothing like garden soil and it is going to take a bit of work one way or another to make it garden soil. Good soil is the foundation of a garden and if you don't invest the work into making good soil, you will be very disappointed in the results.
I definitely agree with the starting small so as not to overwhelm yourself.
If you want to plant in to the clay, it will need to be tilled a couple of times. Put down a good layer of amendments. Along with sand, I would think about compost, well aged composted manure, and anything else organic you can come up with. Till those in, wait a bit and then do the whole thing again, tilling across the grain.
But if it were me, I would do as imafan suggested and make raised beds. Then you don't have to worry about trying to fix the clay. Just fracture and puncture the clay with pitchfork to make drainage holes and build your beds at least 12" deep. Fill them with a mixture of compost and good top soil and you are good to go, no tilling. Build yourself three of those (typical dimensions are 8x4', you want it narrow enough that you can reach everything from the outside) and it will be plenty to get started on the first year. If all goes well, you can keep adding.
If your track is growing weeds, then you can use the cardboard as imafan suggested. The image I have in my head of this track, it isn't even growing weeds. If so, then skip the cardboard, it will just slow down drainage.
Incidentally nice video on the keyhole garden. Applestar had some threads on her keyhole garden. Since search the forum doesn't work very well any more, I could only find this old one:
https://www.helpfulgardener.com/forum/v ... rden#63042