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I use this one right here and it works great. It also lets you compare depth of field from various sensor sizes, so you know what you need to do to make different cameras match.
Just stopping by to point out that my photo is actually a single exposure! It's feasible to do most of the work in-camera and avoid compositing in post.
To take mine, I stopped down to f/16 and used an app to calculate the hyperfocal distance that allowed me to bring everything into focus from the rocks in the foreground to Mitre Peak in the background.
/u/huffalump1 has given a lot of good tips. One difference between the sample photos and yours is the height from the ground. Get low and close to your foreground subject. It will help you balance the frame out a bit and should help declutter your foreground.
You can get a DoF calculator like this one - it helps a lot in developing an intuition about what's going to be in focus, and what not. I'm not saying that everything should always be in focus, but it's better to know how to control it :D
For gunpla photos 80% of the time the answer is "need more light" - and you can get it either by stacking more lamps or increasing the exposure time. The latter is very convenient, but you need to stabilise the camera somehow - tripod is ideal, but books are fine as well :>
The last photo shows what's the problem with the idea of light*boxes*. If you move the diffusing to the lamps (eg. tracing paper) and turn your background sideways, you should be able to fit everything in.
You need to get a Depth of Field calculator app.
The distance from the woman to the bushes seems to be greater than 150 ft. Estimating that the woman was about 30 ft from you, your nearest in focus point is 21.5 ft and the farthest in focus point is around 48 ft. for a in focus depth of 26 ft. Change to f/2.8 and that grows to over 60 feet of focus depth.
This, in addition to the typical sharpness increase from stepping down is more likely the source of your problems.
You have a lot to learn. Start with this DOF calculator.
I use DOF Calculator.
I use an app called DOF Calculator to determine the DOF when I'm actually shooting/filming. It might be helpful for you.