Hi Maurits!
First of all, congratulation and a big thank you for your release manager role! I think it's a really good idea to have two release managers.
I had a good laugh reading your stories about the roles of Plone workers. I found myself very well in them 
Regarding your release intentions: For me, all this sounds good. Another Plone 4.3 and 5.1 are nice bonus, especially if they get the latest security fixes baked in.
Personally I consider everything not running on Python 3 as unsupported. So Focusing on 5.2 is the right thing.
Support 12/2021 seems also a good timespan - if it is the last in the 5.x series.
I only doubt if it's senseful to test for Python 2.7 until 12/2021 - thats almost two years of testing an unsupported Python version. But the promise is to support 2.7 for 5.2, so let's do it.
However, if there is a 5.3 (which won't be AFAIK) I'd drop support for 5.2 by 12/2020.
I like this plan! And I'd also love to have a similar release schedule like Django.