As I'm just about to deal with this next step, did you solve it by storing the LUKS keys in the TPM chip?
I moved to TrueNAS, then full HW rebuild, then I did not like TrueNAS permissions mess (ACLs x filesystem) and finally trialling Windows Server Storage Spaces now, yeah, I know :))
I did massive research back in time, it was hot mess. Maybe now things are better, this looks promising, try to replicate maybe?
EDIT: TPM is not secure. It just covers some cases to make it more convenient. As long as someone is in possession of the server, they will have access.
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Everywhere you encounter tutorials and howtos for luks encryption people will warn you of the consequences when using keys for automated unlocking. It's a important flaw in the whole concept of security. People should simply be aware of that.
Have a look at Windows Bitlocker. It is secure and resists physical attacks. We are simply trying to replicate Bitlocker in Linux here.