Help - STX vs NVIDIA or VOO

Hello, I need to pay my brother some money around $15k. I want to start saving the money in an investment account. I may need to pay him in about 8 months. I recently bought STX, but I am still not as convinced as if it were NVIDIA or VOO - mind you I’m still learning about investment. I feel like NVIDIA or VOO would feel safer? I had some money in my saving account at 3.5% HYSA but I feel like it’s nothing compared to investing the money in stocks. Can anyone help me or guide me? I am using Fid