Hey :GregZ, thanks for filing! There are two sub-issues here I'd like to address separately.
The first is "can we add a way to customize generated passwords?" This is something we've talked about on the team, and it also fixes some other problems (e.g., letting you meet a sites nonstandard password requirements), but it has some drawbacks too. There's are also a number of addons that provide this functionality so a good workaround exists (this might be something you should look into). Our big worry here is that the UI would need to be fairly large and complex, and the value add needs to be weighed against the number of people it might scare away from the feature. I think it's important that everyone uses a password manager, so that's something we have to worry about.
Your second point is "all passwords being the same length reduces PW entropy." This is true, and a good callout, but our password generation algorithm currently provides 87 bits of entropy. This is on top of the fact that each password is associated with only one domain, so this should be enough to feel comfortably secure. But if you want more, those addons I mentioned earlier are still a good solution.
If you're curious, this is the security review that was done on our password generation: https://docs.google.com/document/d/1ox3w8bRDE-x3LxGQbFT3N6SAiIBBdFbmVsv06JlybSI/edit#, and a discussion on Chrome's PWs (the 69+ implementation is different but similar to ours) https://security.stackexchange.com/questions/190796/chrome-generated-passwords-not-high-entropy/190817#190817
TL;DR: we're thinking about this feature but haven't made up our minds on the cost/benefit yet, and don't see the current implementation as a security risk.