Using Photo Stream does mostly work.
Photo Stream images from the phone do end up in my iCloud Photos Library. I assume that either my main iPhone or Mac grabs the photos and adds them.
If the devices did not sync for > 30 days, those photos probably would not be synced to iCloud. But that's not a huge limitation.
The only major annoyance is that Photo ...
I ended up following ankii's suggestion and just chancing it by turning iCloud Photos back on and letting it upload 100,000+ photos.
On the plus side, it did sync successfully without duplicating any photos.
On the minus side, I started it on 09/08 and it just now finished on 09/17. So that's 9 solid days of "uploading". It was on a mostly idle iMac with ...