How to practice system design without a study partner?

I'm looking to ramp up my system design interview prep, but I don't have a study partner, and the paid platforms for mock interviews can get quite expensive. How do people effectively practice system design on their own? My current approach involves picking a design problem (like 'design Twitter') and then: 1) writing out a detailed design document as if I were presenting it, 2) recording myself explaining the design and then reviewing it to catch areas where I ramble or miss key points, and 3) comparing my design to well-known solutions or articles on the topic. I also try to whiteboard it on my iPad to simulate the drawing aspect. Are there any other solo prep strategies that have worked for others? Specific tools for self-assessment, or ways to get feedback without a dedicated partner? I'm open to anything that helps build that muscle.