iMovie runs out of application memory importing clips

Yes, I think that iMovie 9 is the problem. It is not compatible with iMovie 10 on Mojave or later. Nor does iMovie 9 have a library. It has an iMovie Events folder and an iMovie Projects folder. If you open your iMovie 10 app and do File/Update Projects and Events you should be able to update all of your iMovie 9 projects and events into the iMovie 10 library, where they can be played, edited, and exported. They will also still remain intact in your iMovie 9 system. Before doing the update procedure, make sure that your iMovie Events folder and iMovie Projects folder are both located in the Movies folder on your new Mac. Then you should be good to go.

-- Rich

Open the folders in the Movies folder that are labeled iMovie Events and iMovie Projects. If you see a marker in either or both of the folders titled UpdatedToiMovie 10, delete the marker in both folders and try updating again.

If still no luck, it is likely that the folders contain legacy format clips that are not compatible with iMovie 10 following the OS upgrade to Catalina or later, that would explain why the update to iMovie 10 cannot be completed. Here's a link that describes the legacy media issue and lists the media formats that work and do not work, and some comments about converting.

About incompatible media in iMovie for macOS – Apple Support (AU)

As a workaround, since you still have the old iMovie 9 app on your older computer, you can share out the iMovie 9 projects to your desktop as standalone movies, and then import the movies into the iMovie 10 app on your newer computer. The movies can be placed into iMovie 10 projects and edited, although to a limited extent. You couldn't remove some embedded edits in the movie, but you could cut the movie into segments; delete, trim, or move segments, add other clips, add or change the audio, etc.

-- Rich

There is no import size limitation, either for minutes, total size, or number of files. You are either running out of free space on your internal drive (so that iMovie is unable to function) or on whatever drive on which your iMovie library is located (so you have insufficient space to receive the movie clips). The solution is to free up some space.

— Rich

Close iMovie and reopen it while holding down the option key. In the box that appears, you will have the option to have iMovie open in a newly created library. Open in the new library and try to import into it.

— Rich

As long as the library still works for thing other than importing, such as for playback and exporting, you are covered because you can import future videos into the new library. I assume that you are talking about iMovie 10 and not iMovie 9.

-- Rich

That should work. You would open iMovie in the new library on your internal drive and update your iMovie 9 projects and events into that new library. Thereafter you would import any new video clips for iMovie 10 projects into the new library.

-- Rich

You should store your iMovie 9 Projects folder and Events folder together at the root level of your external drive (i.e, not in any other folders). That would make it so that iMovie could find them if the need should arise. Otherwise, you can store them anywhere you want, as long as the drive is formatted Mac OS Extended (Journaled) or APFS. Reformatting erases data, so if you need to reformat first back up your data.

-- Rich

If you can import into the new library, but not the old library, that means that your old library is corrupted, or that it’s located on a drive that has insufficient space.

— Rich