@eclipse
I share your desire for a solution where you can move iMovie projects into another NLE program like Final Cut X, Premiere Pro, Resolve, Vegas Pro, KdenLive etc to leverage the power of each computer hardware. Meaning that your project can live in different computers running different operating systems, but you can edit, render and export projects with minimal fuss. And while XML help achieve cross platform sharing, it requires some work, some restoration of timeline, audio work and so forth if your project has some complications. XML was meant to allow universal portability of NLE project so you can edit and create a movie in one program, then export it and re-import it into another NLE for color grading and final production. XML isn't meant to be a drop and forget solution. Meaning that what is created in iMovie and can be dropped in KdenLive unaltered and unchanged on the timeline as though it's plug and play. Unfortunately, you will not find it with XML. I tried it myself already last year and came to the conclusion that it requires "TIME" and "WORK" and I had decided to standardize the NLE across all available computing platform and also, like iMovie, is free of charge. That NLE is Davinci Resolve. Why did I choose Davinci Resolve other than it was free for the lite version?
Davinci Resolve is available on 3 OS platforms; Mac OSX, Windows and Linux! Very helpful if you plan to build a Xeon based multi-core super duper latest Nvidia GPU render farm for brute force encoding your projects in h.264 and h.265 with a Windows machine or a Linux machine.
My solution to the iMovie projects conversion is to export the project to a standard XML format and import them into Davinci Resolve. There, I work on the timeline and make the project good enough to be saved in the native Davinci Resolve format. Once saved in Davinci Resolve format, you can then export your Davinci Resolve project as a Project Archive file. This project archive is similar to the iMovie library file where basically the archive is a backup of the entire timeline, transitions, titles and original footages. When you import this archive say in another platform like in Linux or Windows running the Davinci Resolve NLE software, it will restore the entire project with zero fuss! You can literally work in any platform as long as it is able to restore the project archive. In fact, I set up the Mac Mini as a media server for Davinci Resolve and Resolve is designed to work natively with networked media drives. When your designate one of your storage drive as a network drive, it will seek that drive during launch.
So you can edit in Windows (I have a Windows 10 machine), edit on a Macbook Air or on a Mac Pro.
Why did I chose Davinci Resolve? The reason is simple. I can leverage the pluses of each computer I have. For example, I have Quicksync for fast h.264 encoding, but my Mac Pro does not have quicksync and I don't have a Vega 56 to use hardware encoding in High Sierra. NO PROBLEM!
I leveraged the power of my Mac Pro 5,1 with the Radeon RX580 and edit my iMovie footages XML exported in Davinci Resolve and then work on the titles and transitions and effects very smoothly. Then when I export, I export it to my network server as a project archive file with all the original footages etc.. Lossless which is pretty quick. Exporting to Pro Res is actually slower than exporting to project archive. Then using my Macbook Air and restore the project archive. Plug and play and then I just render that footage I did with my Mac Pro with my Macbook Air using Quicksync to a h.264 file.
For a 1hour 4K to 1080pHD export, it took 51min with just the iGPU. If I have an eGPU for the Air, the process would be much quicker because the titles and transitions and LUTs need to be re-rendered on the new computer's GPU. The Air's iGPU is severely underpowered than the Radeon RX580
For the same 1hr footage export, it will take the Mac Pro 1hour and 51min with a Radeon RX580.
It's cool to see my Macbook Air churns away on those h.264 projects while I edit the next imovie import project on my Mac Pro 5,1. Davinci Resolve supports batch export as well. I recently re-did some of my older movies; improved some of the footages by removing vibrations with VideoProc and improving sharpness and upscaling from 480p to 1080pHD using Avidemux. The horsepower provided by my Mac Pro 5,1 is well appreciated coming from an older 2011 Mac Mini.
This means that if I switch or add to a Windows or Linux platform, then I can still access my video archives and edit with minimal fuss. I can also build a low cost fast Windows multi-core system with the latest Nvidia GPU card to speed up the rendering if I need to.
So basically, you have to decide on which NLE you want to standardize on with the most flexible support and options and go from there. It's not easy to just move from iMovie to any platform and then expect that it is a plug and play experience. It's usually not. But once you ported the iMovie project into a NLE software that can be used universally across 3 platforms, then your options and choices become better.