Is there any disadvantage of using the device PIN also in the passphrase, if you only type the passphrase in on your device and never on the host system?
I think the only slight disadvantage of using your PIN as your passphrase is that if someone knows your PIN, he may guess your PIN as passphrase.
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Then it should be ok.
Because using the PIN in the Passphrase requiring no additional memorization effort and risk of forgeting so i can just add it as a security improvement.
I also would add 2 or 3 random words to the Passphrase.
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I think Andreas Antonopoulos used to recommend 6 to 8 words in some of his videos as a good security practice.
Anyway, definitely write that somewhere on paper (separately from your Wallet Backup). Many people around are desperate, that they can’t remember even if they have a hint words, they are not able to recreate it after few months. Assume it will happen to you and protect your self before you start to use Passphrase. Assume accident and loss of memory.
I would test load the passphrase daily to not forget it.
Especially if its the PIN of my Trezor, i can unlock it every day just to memorize the PIN.
Personally, i want to protect not only against obvious examples like car accidents, brain bumps but there are others. Like Covid is mostly forgotten, but my personal experience - for nearly 2 years i entered PIN to arm/disarm the building nearly every day. So attack vector here was Covid & Brain Fog and i remembered the numbers, but i was entering PIN backwards. So another EXP gained here 
Or heritage is its own chapter to avoid brain-only storage 
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Well, this is self-custody, own responsibility, security and your rules. It’s really up to you how deep you go and what are your personal experiences.