Can FCP read timecode from Sony FX3A MP4 container?

Terry, FYI the default action of QtChange is to make this change. IOW you don't have to pick any options; it just does it. That utility can do many things and handle various timecode-related issues, but to "tweak" a Sony MP4 file header to enable FCP to read timecode, it just does it.

It has the feature because years ago, I saw this need and asked the QtChange developer to add this feature, so he did.

But do NOT do it on already-imported material. Otherwise, FCP will detect the clip has changed, and it will go offline. It must be done before the clip is imported.

While using the QtChange utility this way seems safe, and I've never seen it cause a problem (the above scenario notwithstanding), it still updates the media file. For safety, it might be better to duplicate the media files beforehand. That raises the question: if you're going to do that, why not just rewrap the files with EditReady?

On APFS volumes, macOS automatically uses block-level data cloning when you duplicate a file. Thus, the duplication is very fast, and it doesn't take up any more space (initially). Under the covers, the APFS filesystem maintains only a single copy of the file and presents the illusion of two separate files, the original and the duplicate. As changes are made to either file, the contents begin to diverge. As that happens, APFS tracks changes at the block level and invisibly stores only the changed blocks.

The APFS data cloning behavior is not available on HFS+, ExFAT, or NAS drives. It's a good idea to use APFS on all Mac drives -- including mechanical spinning drives. The former advice to use APFS only on SSDs is outdated; it should be used on all locally attached drives.

On recent versions of macOS, APFS has a significant performance advantage for certain file enumeration operations on mechanical drives, the opposite of the old advice. There are cases where a file/open dialog appears on an HFS+ mechanical drive with thousands of files, and it hangs for a significant period with a "beachball," whereas the same drive using APFS avoids that. I don't remember when this change happened, but I first noticed it on Ventura. A colleague and I tested it repeatedly on multiple disk subsystems, Macs, and macOS versions. I examined XCode Instruments traces to verify it was a real change.

So while experience shows using QtChange to enable FCP-visible timecode on Sony MP4 files is very fast, safe and conserves disk space, a more conservative procedure might be rewrapping the file using EditReady before importing. While slower than QtChange, it is faster than transcoding.