There is no way to preserve folders and photo order when importing to the Photos app in iOS (or the Mac for that matter). Not that I know of. If there is a tool to do this, it’s probably a part of the iMazing app. I’m not that familiar with all it’s capabilities.
Photos are automatically organized by date using the photo’s meta-data. If you have older photos without metadata, it will use the file creation date. Or maybe it’s the edited date. Not sure.
Photos has albums, but these are not actual folders. They are functionally lists of photos. The actual photos is tagged as belonging to that list, but it’s location is not saved nor is the photos duplicated.
This all works pretty well for photos taken or added to the Photo album individually, but as you noted it has issues when importing a large collection of already organized photos.
If I were in your shoes I’d probably try importing the photos to the iPad one folder at a time, then creating an album in Photos and adding them from the Recents album to the new album. It might be possible to automate the album transfer part to some degree using the Shortcuts app.
There are also a variety of third party photo album apps available. If you don’t mind losing the convenient integration with iOS that Photos gives you, you could see if one of them will do the trick.
Of course, you could just import the entire folder structure to iCloud Drive. You would have to pay for the extra space, and they will not show up in the Photos app; but they will be on the iPad, and viewable as individual photos in the Files app.
Afraid that’s all I’ve got.