Blastoidea I recommend reading through https://grapheneos.org/faq#encryption in case you haven't already.
A 6 digit PIN doesn't really provide meaningful protection in cases where the phone doesn't have a proper secure element, but Pixels do. The secure element throttles attempts to unlock the phone:
Standard delays for encryption key derivation enforced by the secure element:
0 to 4 failed attempts: no delay
5 failed attempts: 30 second delay
6 to 9 failed attempts: no delay
10 to 29 failed attempts: 30 second delay
30 to 139 failed attempts: 30 × 2⌊(n - 30) ÷ 10⌋ where n is the number of failed attempts. This means the delay doubles after every 10 attempts. There's a 30 second delay after 30 failed attempts, 60s after 40, 120s after 50, 240s after 60, 480s after 70, 960s after 80, 1920s after 90, 3840s after 100, 7680s after 110, 15360s after 120 and 30720s after 130
140 or more failed attempts: 86400 second delay (1 day)
If you don't want to rely on the secure element, you can go for something like a 7 word diceware passphrase instead, but for most people, a random (and this is important) 6 digit PIN is sufficient.