You are missing quite a bit.
First, you need to rotate passwords from time to time. That leads to exceptions in your rules after some time. You will end up with unchanged passwords for sites where they don't expire, sites with a 3rd version because the passwords expire after 90 days, sites where they expire twice an year, and so on. Sometimes you will forget the password, and the site will not accept the previous one. You will have to keep track of the password count.
Some sites require special chars, some don't allow special chars. Some need a password at least 8 digits, others require passwords 8 digits or less. There's no way to accommodate every site in a single rule.
No worries about hacks from a revealed password
No, you have to worry a lot. When that happens, all your passwords are at risk. If I see
12f34a56c78e90abcdef, I can tell your GMail password is
12g34m56a78i90abcdef. Someone targeting your accounts will have an easy task deducing your other passwords by trying to login on interesting sites with your scheme in mind.
Now if your password leaks and it's
4+YIhLy4lBFXOmCYhbELxnABr/UAbjmK (generated by using
dd if=/dev/urandom bs=64 count=1 | base64 -w0 | cut -b1-32), it shows 2 things: you use secure passwords, and you use different passwords because nobody will memorize that string.
The issue I consider the most significant is the mental load required to manage this scheme. You may find it small at first, but when you have 100+ services with dozens of exceptions, it's tempting to just fall back to the default behavior and reuse passwords.
Using a password manager reduces this load considerably. You can have a master passphrase with 10 words, and use it once a day. And have a 6-digit PIN to unlock the password manager that you will use several times a day.
Using a password manager have risks, sure, but I consider those risks way more manageable and bearable than not using a password manager. Remember the AviD's Rule of Usability:
"Security at the expense of usability, comes at the expense of security."
So if your password management solution is secure but not usable, it won't be used.