The model and form factor create significant obstacles for you in your goal. For one, you're going to be limited to single-slot GPUs, so a GTX 1070 is right out. Second, you have an oddball formfactor PSU and proprietary connections, so upgrading that will be difficult. Third, you can only get 25W from the PCIE slot.
You chose well in getting a GT 1030 into an old small form factor office PC. But this is about as far as you can take this rig without changing out the motherboard, power supply, and case. Given this set of contraints, you already have the most powerful GPU you can get right now.