I need a solution for how to work with Lr on iPad and LrC.

"Export them all as DNG (to keep the edits.) Then import the exported files."

There is no need to do that. You can convert images to DNG right from within Lightroom Classic.

"Syncing from LrC isn't a solution because Adobe, in their infinite wisdom, decided not to allow LrC to sync full resolution files."

Lightroom on the iPad does not need full resolution images if you want to edit them. That is exactly why Adobe chose smart previews. You sync small proxies instead of full size images that would quickly fill up your cloud space, and still you can edit them as if they were originals.

"I'm pretty sure I said that I don't want to edit on PC. I've switched to fully editing on iPad. And unless I am misunderstanding, having a more expensive plan won't make LrC able to sync full res files."

No, but it would allow you to keep the original photos in the cloud. Right now you delete them when your 20GB cloud space is getting full. So lack of cloud space is your real problem, not the fact that Lightroom Classic syncs smart previews.

"If only LrC could sync full res files, I would at least not have to work through my PC to get my final jpegs. I'm so frustrated. Why does Adobe make it so hard to edit long term on iPad?"

Adobe does not make that hard at all. There are quite a few people who only use an iPad and do not sync it with a desktop computer. The only thing you need is enough cloud space so you don't have to delete originals from the cloud in order to make room for new images.

P.S. Consider switching to the Lightroom plan if you have to sync to your computer for some reason. Lightroom (non Classic) does sync originals to the cloud.

My thoughts-

"Then I open Lightroom Classic and wait for the photos to sync."

Good! Then all the files are local and in the LrC Catalog.

"Export them all as DNG (to keep the edits.)"

Why bother? All the edits are stored in the Catalog. And you do make Catalog backups of course?
"Then import the exported files."

Why bother? as above- you have the original files AND their edits already in the Catalog.

"Then delete the synced files from the cloud and iPad."

Well yes, that is one way to keep within your 20GB, but if you want Full-Size files in the Cloud then you would need to buy more storage.

I am quite satisfied editing the proxy (Smart Preview that do not use my Cloud storage) files on my mobile devices, the only limitation being that I cannot [edit in Photoshop] or [Share/Export] fullsize files from the mobile devices. (I can, using the fullsized downloaded files in LrC).

What do you guys do?

Me!- Lightroom-Classic is my 'Master' editor. I Import, edit, sync (proxy files) to Lightroom from selected Collections.

Only photos taken on my mobile devices are imported to Lightroom-(mobile) as full-size. These sync down to LrC.  I use very little of my Cloud storage allocation.