I've been editing videos for a while, mainly for personal projects and some freelance work. From my experience, Davinci Resolve is actually pretty solid for syncing separate audio tracks with video parts, especially because it has powerful audio synchronization features. It's user-friendly and free, which is great for handling concert recordings like yours. For making the screen go black between clips, you can easily add black video clips in the timeline where needed. Regarding video aspect ratio changes, I use Movavi Video Converter. It's handy for adjusting videos to fit different screen sizes or platforms. Movavi makes it easy to change the aspect ratio by just adding your video, choosing your output format, and then selecting the aspect ratio you need. It's a straightforward process and helps ensure your video looks good on any device. Check it out
here for more details.