I have a ~22k pic iCloud photo library. I want to have a folder of ~1.2k pics available on my iPad in full resolution while offline, all the time. Can this be done?

Hi - I have had iCloud photos turned on for about 2 weeks. My old iPhoto library of around 22k pics uploaded no problem. My issues are with now viewing them on my iPhone and iPad, which are set to optimise storage.

On the iPad I used to have a folder of about 1200 pics which I'd use as a photo frame by chucking on as a slideshow on my desk at work. I don't connect to the wifi there. Since iCloud photo I can't do this, as the pics are mostly low res, if there at all (two weeks after upload the vast majority of my library appears as just a sea of grey squares on both i-devices - thumbnails have not appeared for huge swathes of the library at all. Is this going to right itself?). I read about using Favourites to ensure local full res copies are kept, so I've gone through and favourited each pic (no option to do this in bulk on the iPad?) and this has ensured at least that each pic of those selected have thumbnails now, but no guarantee that the full image will display. Even when I run the slideshow connected to wifi the low-res pixelated placeholder is what's displayed for most of the time the image is on the screen. And if Favourites is the way to force local hi-res copies I still have no need for these to be also stored in full resolution on my iPhone.

Basically, I want a way to force folders to be available in full, offline, and specific to the device. Is this possible? Big step back for what I use this thing for if not.