Re-turf a garden that is soft/sinking?

Hi all,
I'm planning to sort out my back garden as it is weed infested!
Here's the real problem tho - the ground is so soft & unlevel. If you stand in one place just for a minute, you start to sink!
The house is a newish build (3 years), so I'm hoping the garden wasnt done properly?
Now this is what I'm planning doing:
1. sprayed the whole garden with weed killer (it will kill everything) - have this this today.
2. wait a couple of weeks for everything to die
3. remove the top layer of dead weeds/grass
4. Rotovate the ground
5. Lay some new soil if needed
6. Rake until soil is fine & level
7. Lay new turf
Is this a good plan? BUT how do I address the issue of the soft/sinking ground?
Thanks a lot!

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