I'm also having issues regarding updating my Xperia XA1 Plus. This has been ongoing for more than 2 or 3 months now. I found a solution but this isn't permanent in my case.
- Remove google account
- Go to Apps & notifications -> App info. Click the 3 dot and select show system
- Clear cache and data of Download Manager, DownloadAgent, Downloads
- Restart Phone
At first this will work, but eventually it won't still update all my applications. I even tried your suggestion to revert apps to internal storage, but this doesn't help too. Please help me. I'm not sure if doing a factory reset will fix this and I'm also worried that it might cause my phone's warranty to be void. Thank you very much.