While practicing in isolation with a friend is very useful up to a point, at some stage in your preparation process it’d be time to face the music. You need to get actual real-life interview experience under your belt.
Mock interviews are a fantastic way to practice system design questions. System design questions are all about organic discussions around trade-offs. Try to find an unbiased and knowledgeable person, and do several interviews with them. It can be a friend, a colleague, or you could use a paid service.
Paid mock interviews allow you to practice being comfortable discussing ideas with a stranger, working under the stress of time pressure, and guarantee you objective feedback. And if the person providing it is skilled at interviewing, you’d get lots of action items and high-quality advice.
If you choose to do it with a friend, make sure they stay impartial and give you honest feedback (friends tend to be too nice to us). The nice thing about doing mock interviews with friends is that they are very generous with their time, so you can do multiple interviews with them at no cost. But make sure you choose a skilled interviewer: there is little value in doing mock interviews with someone who has poor interviewing skills.
Nowadays, is it also possible to do mock interviews with other people online for free. One service that we found useful for this is Pramp.com - you could use it for your system design interview preparation.
No matter if you choose paid services or interviewing with friends, mock interviews are going to be highly useful to you when it comes to system design questions. Make sure you do at least a few of those prior to your interview.