I've been using Krusader for a few months after looking for something similar to Double Commander which I used years ago, It's been mostly great but I've been having a few issues mainly when transferring or archiving larger files.
I've found making zip & tar archives with in Krusader takes a lot longer than doing so in Dolphin, Also I've recently been having issues when trying to copy or move large amounts of data to separate drives, last night I had a transfer fail on me twice then take a lot longer than expected when I finally fixed that issue.
I've already looked back at double commander and some of it's forks but stuff like most of them lacking a dark mode has been putting me off using them regularly. I'd ideally like it to be Qt based to it fits my KDE theming, But running a GTK program isn't a deal breaker for me.
Any advice or ideas would be great thanks in advance!
I'm not sure if it meets all your requirements, but Dolphin has a dual-panel mode if you press F3 and has lots of other configuration options as well
I actually didn't know this! feel abit silly now but I'll check it out thanks :)