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I pay the adobe subscription so all my photos are in the cloud, which stops my iPad from getting clogged up. And then I use Adobe’s cloud downloader to back them all up to an external HDD using my PC.
If you don’t want to pay for the cloud then I can’t see an alternative to exporting everything to HDD and then deleting it from the iPad - which would work, but would be less automatic.
With Lightroom specifically? Not really. You could export things straight to the external drive and then delete them, if you wanted to keep the iPad storage clear. Or there’s an Adobe app you can use to download your Lightroom library to an external drive attached to a PC, if it’s the backup you’re after. I don’t know how it is with all the possible photo editing apps out there, but my general experience has been that you have to import things to the iPad app and work on them locally, but you can export them wherever you want after that.