How do I find a high-quality video converter for .m2ts files on my new Mac Pro?

Thanks for the screenshot, but the "More Info" section needs to be expanded.

Here's an example of the kind of info that will be there:

Also, I looked into video formats that Photos supports and .mt2s is not one of them, so that explains why you are unable to import the videos into Photos. You were on the right track, if you really want to import them into Photos you need to convert them to something Photos supports. While .mov is Apple's official container format, I suggest MP4 for futureproofing in case you ever want to move, copy or share the videos to another platform.

I still suggest Handbrake as the best option. It is widely used and respected among the pros & video enthusiasts in the Final Cut & iMovie forums.

Timneedshelpnow wrote:

What is a higher quality ... .mov or .mp4 ?

Neither. Both are "containers" not video formats. They can contain different kinds of video & audio formats.

I'm wanting to convert from .m2ts. (If I remember correctly that was my Sony Camcorder.)

.m2ts is also a "container" that is used for BluRay discs & AVCHD video. Your Sony Camcorder is probably recording in AVCHD. It would help if you identify the specific camcorder model.

Regarding conversion ... why do you want to convert the camcorder videos and what format do you want to convert to?

Most people here would probably suggest Handbrake.

In the Finder, select one of the video files. Do GetInfo (CMD-I or File > GetInfo). Take a screenshot of the General & More Info sections of the GetInfo panel. Post it here so we can see information about the file.

What is the error message when you try to import one of the videos into Photos?