I guess I’m not understanding how this is functionally different from what I already am doing. Why would your 12 character solution be more secure than my 14 character example

Yeah, it's because it's close to the associated domain. The way I see it, this bastardization adds little entropy (there's only so much possible variations) but also rather easy to forget. And a huge problem, in my opinion, is it's using your mental capacity for per-site suffixes rather than master password.

A possible attack I see, is if I set up a site, say a forum called MyLittlePony.su with no password protection whatsoever, and lure you to register on it. If I scroll through the accounts and notice your password to be "hunter2MyLittlePenis", I might go to paypal and give it a shot with "hunter2PenisPal". Or, somebody whom I sold the database to, might. It's extremely rare that anyone would even look at your password specifically unless you are some kind of celebrity, but it's still a possibility. Maybe some future AI tech would be able to crack your strategy (I've tried, ChatGPT told me to fuck right off and FreedomGPT is not good enough yet)

Though you've said you also keep notes, which deals with the easy-to-forget part of the problem, so my first thought was to get rid of bastardization and add fuck-all amount of entropy by using a truly random suffix. That'd deal with the above problem. But, that'd mean that it's your master password that is the suffix now, and you wouldn't be able to access sites without the notes at all, hence it'd be easier to go with password manager at that point.