Hey, welcome to the community!
You have near infinite options for GPU, I would start by determining the amount of performance you need (aka what games at what resolution and refresh rate you plan to play), and set your sights on a GPU first, then base your build around that.
As for the case:
-You could go ultra compact with a 4L case such as the Velka 3, Densium 4, which would limit your GPU to something like an Aero ITX 3060ti (still a MASSIVE performance jump from a 970).
-You could go mid-size SFF, generally around the 10-11L mark mark with options like the FormD T1, Dan A4-H20 which offer full GPU length support and thickness up to 3 slots.
-Or you could go with something like the Meshlicious/Meshroom, approx 15L, which will give you virtually zero restrictions in terms of GPU compatibility with support for card up to 4 slots in thickness (at the expense of CPU cooler height).
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