I am trying to make a solid fused piece that would have a shape in the center. I suppose you could make the shape in the glass with a fuse step and then tact fuse a top and bottom on to it. I am worried about losing some features and having the glass collapse.
I've never tried this, so this answer will be largely speculation based on the physical properties.
You can leave a void, but I don't think you would be able to make the void a complex or delicate shape. There are a few ways to start with a well-defined void: engrave it into one or both mating faces of two pieces; start with three pieces and cut the shape out of the middle piece like a stencil; make the shape using a glass tube and seal the ends to trap the air, then embed the tube in a bed of glass dust and fuse the bed (this wouldn't be applicable for a 2 mm final thickness).
With all of these approaches, glass fuses by becoming a liquid and flowing. The molten glass will flow in a way to minimize the surface area of the void. It will fill thin areas first, forcing any trapped air into bubbles, so you will lose the shape.
You could embed a specific shape in fused glass by making the shape out of a material that melts at a higher temperature. However, that wouldn't be a void.
The only way I can think of to have a void of a specific, desired shape is to not fuse the glass. Use one of the methods, like mentioned above (e.g., engraving or stencil), to create the void, then bond the pieces together with clear adhesive that bonds glass.
If the goal is the appearance of fused glass, another approach would be to fuse the glass. Then engrave the shape in the back side.
Another idea that might come close to your objective. Create the void as in the gluing approach. Fill the void with fine dust of colored glass. Then fuse the glass. Colored glass dust would melt, and it might hold the shape, and at least be a contrasting, embedded shape.
A fully encapsulated void in thin glass is difficult to impossible to do hot in a kiln. A carved out indentation open on one side is trivial. You could the encapsulate it by cold-fusing (i.e. gluing) another sheet of glass to the back.
For a design such as you show I’d kiln carve it by cutting the design out of ceramic fiber paper, placing a double sheet of glass over the fiber paper in the kiln, and firing to a full fuse.
Trying to tack fuse a cover into this will have problems because the tack fuse temp is high enough that the glass will move and distort the carved pattern, and because trapping air in the pattern is likely to cause bubbles. You can avoid these problems by filling the pattern with enamel or powdered glass to form a sold block. Either way will end up thicker than the your 2mm final size by default, but you could create a blank for pressing this way. The pressing will likely distort the image some.
Bullseye has a TipSheet on kiln carving at https://www.bullseyeglass.com/methods-ideas/tipsheet-1-kilncarving.html
Another option (similar to previous suggestions of engraving or acid etchjng) would be to sand-blast the pattern. It really depends on the effect you’re looking for and the tools you have available. You could get your single layer look by sandblasting, filling the sandblasted cavity with a low fire enamel and then firing to a temp that’s high enough for the enamel to mature but low enough that the glass doesn’t deform.
If I understand you correctly, you just want to cut out the hollow of that shape in your glass square.
I would just use a Dremel rotary tool with grinding bits and simply carve out the design. I've carved glass before with a Dremel - it's slow going, but it works. You'll need different grade bits, starting with coarse and then with finer grits, finishing with rubber polishing tips to get it super smooth.
But with the dremel, you can use whatever kind of glass square or shape you want.