I have no idea what TPM is

Read Skull giver's reply or look it up.

Re-reading your post, I take you want to avoid typing long and tedious password? And that's why you want to auto-decrypt?

1. (Recommended) You could use strong memorable passwords that are not difficult to type and enable autologin. Related xfcd comic:

2. systemd-cryptenroll: For TPM usage, I highly recommend using secure boot. Though not sure if you can easily do that. A less secure alternative using systemd-cryptenroll would be use tpm2-pin and bind key to no pcrs (discouraged). But then you'll have to use luks2 for encryption. Notice from man systemd-cryptenroll regarding tpm2-pin:

Note that incorrect PIN entry when unlocking increments the TPM dictionary attack lockout mechanism, and may lock out users for a prolonged time, depending on its configuration. The lockout mechanism is a global property of the TPM, systemd-cryptenroll does not control or configure the lockout mechanism. You may use tpm2-tss tools to inspect or configure the dictionary attack lockout, with tpm2_getcap(1) and tpm2_dictionarylockout(1) commands, respectively Also tpm2-pin is not disk encryption password and short alphanumeric password needed so tpm decrypts the device; so encryption password should be secured in a safe place. Also check if your distro supports systemd-cryptenroll.

3. usb drive: read previous comment

4. clevis: It probably isn't as simple as systemd-cryptenroll but I guess you can use zfs and combine that with tpm2-pin if not using secure boot (discouraged).

You'll have to make a compromise somewhere between security and convenience. Even if you use pam mount, you'll have to enter the password, biometrics won't do.

Edit: remove unnecessary user tag and add img uri