iMovie on iPad: How can I convert an AVCHD video to work in iMovie? (Preferably with ffmpeg)?

I have some non-Apple videos I want to edit in iMovie on the iPad. They are AVCHD (.MTS) videos from a Sony RX100 series camera.

What format should I convert these to, to work well in iMovie on the iPad? I'm very familiar with ffmpeg and can use that. But I have no idea what exactly would make iMovie on the iPad happy.

Some details: Photos would not import these as is. I tried re-containerizing them from .MTS -> .MOV using ffmpeg. Then the progressive (non-interlaced) ones at least imported to Photos. But even then, the iMovie project acted weird-- it would stop playing when it got to one of these clips. What do I need to do a video, to make it possible to zip around the timeline in iMovie without stalling / hitching problems?

PS, the iPad is NOT an iPad Pro, it's a regular iPad-- just noticed I set that tag wrong, but can't seem to edit it now.