You can embed a minimal ssh server (like dropbear or tinyssh) in your initramfs to remotely enter the passphrase. There are some guides for that online

Yes, this works. I have it set up for some servers that have encrypted hard drives.

If you can fiddle around and get the VNC functionality working, then as long as you have it behind a bastion host you basically have insta-BMC.

I kneejerk-recommend a bastion host because a) Intel ME, and b) there was that all-0 password bypass a while back and uhhh yeah who knows what other "sneeze correctly and the leaf falls over" knock sequences there are.

SSH port forwarding, VPN, RPi running the remote-access AMT web UI thingy (runs on Linux, forget what it's called) behind nginx with authentication... you get the idea.

You could use a Mainboard with an IPMI module and KVM-over-LAN functionality. This way your are able to do everything remote, including mounting isos to install a new system and ofc you could use it to enter your password for the encrypted partition.

Store the LUKS crypto key in the TPM, and make sure secure boot is used so the TPM will only reveal the key if the systems boot chain is verified.

Nope. That's dangerous, TPM keys can be sniffed on the bus: https://trmm.net/tpm-sniffing/

Is a LUKS key being sent remotely to Intel ME _not_ vulnerable to being sniffed on the LAN?

You don't need to involve ME into remote decrypting. Embed the LUKS binaries and dropbear into your initramfs, and off you go.

I have followed this guide: https://blog.buettner.xyz/luks-verschlusseltes-ubuntu-20-04-...

look into tang and clevis.