TWRP is by far the most popular custom recovery, and it can do everything from installing ROMs to backing up your entire phone. For TWRP to access the internal storage, it has to support the encryption method used by whatever Android version you have installed. If you're already rocking a custom ROM based on Android 9, you'll be happy to know that the latest version adds decryption support for Pie.
The version with Android 9 support is 3.2.3-1, and it's now available for select devices, including both generations of the Google Pixel/Pixel XL. Decryption works even if you have a lock screen PIN or password (as long as you enter it when booting into recovery). You can download the proper image from TWRP's website, or through the official app linked below.