How to practice system design without a study partner?

i'm trying to seriously ramp up my system design skills but i'm finding it tough to practice effectively without a study partner. all my friends are busy, and paid platforms for mock interviews can get pretty expensive, especially at a startup salary. i've been trying to write out full designs in a google doc, including diagrams and detailed explanations for components and trade-offs. i've also considered recording myself talking through a design. has anyone had success with solo system design prep? are there any specific tools, prompts, or feedback loops you've found useful when you're just working by yourself? i really want to improve my ability to articulate and defend design choices.