The native Files app on iPadOS 14 should be able access to an APFS-encrypted device. The encrypted SSD should be displayed on the sidebar of the File app. You can tap the icon of the drive and enter the password when prompted. If the iPad does not recognize the SSD, it could be caused by system errors.
According to your description, your MacBook can read the APFS-encrypted SSD while the iPad Pro can't, however, another encrypted HDD is readable on the iPad Pro, it seems that both the iPad and the SSD function well, there could be incompatibility between the SSD and the iPad.
You should keep the iPad Pro updated to date. Outdated iPadOS may cause the encrypted SSD unreadable. Go to the Settings on your iPad to check for updates, some users found the encrypted hard drive is readable after updating iPadOS.