My own plan is somewhat similar. There is no practical way for me to migrate from Aperture to anything but Photos.app --- far too much metadata is lost (album/project relationships, etc).
Not to mention complete loss of thousands of image edits! Life is too short.
Indeed, even with migration to Photos.app I'm sure metadata is lost. Heck, when I went from iPhoto to Aperture I lost the notes I'd made on albums (never supported in Aperture).
Sure, Apple could have worked with Adobe to provide a tolerable migration path to LR, but they chose not to do that.
So my plan is:
1. Migrate one machine to El Capitan after next release and test my Aperture there. I assume it will work well enough but I need to do my own testing.
2. Stay on El Capitan for next 2 years.
3. Periodically use Aperture Exporter to create a reference/archival library that will be about as standards based as possible. I strongly recommend buying that application now. It's $20 Canadian direct from developer. This is a pittance. Given the nature of software development today you should assume AE will not be available in 12 months. So get a copy now if you want to be safe.)
4. Begin using Photos.app to "develop" my RAW images into JPEG but continue to use Aperture for image management. (I'm much more concerned about archival status than I am about future image edit excellence). I think over time Photos.app image editing tech will exceed what I can do in Aperture.
5. Somewhere around 2018 flatten all my keywords (I use Aperture's inheritance model queries -- which were too advanced for market really) and migrate to Photos.app 2018. Assuming Photos.app is still around in 2018.
It would be pleasant to switch to something other than Photos.app, but I don't think the prosumer photo market is very healthy. I wonder even about the future of Adobe Lightroom. I doubt we'll ever see anything with the image management power of Aperture in our lifetimes, but image editing/processing will continue to develop.