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You could use the file FP3-REL-2.A.0134-20200703.120010-userdebug-fastbootimage.7z from here …
… with the guide from here (not downloading the file suggested in the guide) …
Be aware that a downgrade of the major Android version (from Android 10 back to 9) as well as unlocking/relocking the bootloader (included in the guide) will force a factory reset, so you will lose your data on the phone if you follow this procedure, so backup all your important data beforehand or make sure it is completely synced to the internet.