My old friend,
As you suspected, no matter how its sliced, your will be in some long term reoccurring penalty.
Why not at the very least have ABCDEF Geo target each individual site?
Even with 900 sites it's not that big a issue to create some individuals home page text. looks like you have some value here, so you may want to invest further in your work.
I have seen your kind words in this forum for many years and have seen you as a real qualified SEO based on what you ask etc..
Sometimes that little bit more is worth so much more. Not only will you increase access at a local level but as things grow you can integrate a service to allow each dealer in a area to expand their site and be able to attach paid marketing platforms to the accounts individually.
You are creating a porthole that all the rising stars can come back to for expansion. If you do not do it this way, the stars that will rise up and do the big dollars, will move to independent platforms, that may not benefit you in the best way, or the group as a whole. You owning the domains will help incredibly, not to mention you will have control over linking and policy.
If you work your 900 plus clients and encourage localized growth, everyone wins.
Suggested GEO targeted URL :
abcdefTempeArizona.com
That will give some chance of The Purchased URL showing up locally even with all that duplicate content included. You will get enough points to get you some SERPS organically, simply because lack of local competition may prevail and allow a very low scoring site access to some good keywords.
Social media will attach nicely to this type of structure too, and at a local level may help.
Best,
Jay Bale