Here is my solution. I apologize in advance for being long winded.
After looking and trialing some of the alternatives (C1, LR, After Effects) to Aperture I've come to the following conclusion. There are no DAMs as good as Aperture. Lightroom may be good replacement but I will not buy into any subscription model that makes my work inaccessible the day I stop the subscription. Just to be clear, I have no relationship to ON1 other than as a customer.
My Aperture replacement of choice is ON1 Photo RAW 2018. I've been using ON1 products for a few years to do selective enhancements to a few images. The down side here is ON1 Photo RAW is a new product, just over a year old and is missing features found in more mature products. It has bugs and some rough edges. It does not have an automated conversion tool. It is not a DAM but a Browser with some DAM features. Organization is done using the host OS file system. Adjustments are stored in a side car file in the same folder as the image. Metadata goes into another side car file. Optionally you can enable a cataloging function that also stores the adjustments, metadata and previews. I like this redundancy. Loose your catalog and the side car files have your adjustments and metadata. Lose your image and the side car files, restore the image and the catalog will be used to automatically recreate them. The catalog is also used for faster preview display and searching. It also allows cross folder search smart albums across multiple folders.
Currently compared to Aperture ON1 RAW has only basic keyword and search features. It does not currently have a Stack feature. However, using the Browser to move items in the file system automatically handles the side car files. ON1 works on a Mac or Win system.
Since its introduction in late 2017 major progress has been made. After multiple releases in the past year it is close to what I need with fewer bugs and better stability. This program has a lot of potential and I am confidant it will continue to get better. Future features are user driven. It already has features Aperture lacks such as Pano, HDR, and Lens Correction. It is fast! Drag a slider and the results are reflected on screen immediately. It has all the adjustment capability that most people could ask for. It has presets that allow for rapid experimentation, and saving your favorite looks. It has great masking features, syncing, and one of the best resize functions in the industry.
I do not have as big a library as some but migrating 30,000 plus images is a non trivial task. Right now my migration is being held up by a metadata bug that affects only the migration. This is giving me time to get my Aperture Library in order. That means deleting unacceptable images (out of focus, duplicates, just don't like them), review star ratings, and keyword clean up, by eliminating synonyms, words I'll never search for, etc. Additionally I'm making sure all my Projects are well named, organized in Year folders and using Relocate to get the Finder folder names in sync with the projects. The clean up is one Project at a time. When everything is ready and tested I will go Project by Project telling Aperture to Write Metadata to the Orignal. I will also Export full sized Tiff files with the Aperture changes baked in. Last I will open those files in ON1, beginning with my highly rated and adjust them.
Until ON1 improves its metadata features use Aperture to input metadata (location info, keywords and copyright) because it is so much more robust. I'll Write Metadata to Original and do the rest in ON1.
Here is one excellent write up with some tools to help the conversion from Aperture to Lightroom. I read this one and others and changed as needed. It will cost you an email address. It looks like you are buying it but the price is zero. The store is just the delivery vehicle.
https://www.scottdavenportphoto.com/products/migrating-from-aperture-to-lightroo m