@ali_hugo Thank you for starting this thread! It would definitely be good to iterate over the current workflow, and get more collaboration & planning happening. We are definitely lacking on that aspect currently, as a community. 
A few comments about your specific suggestions - I’ll reply by my order of preference of the suggestions 
Give ‘Core Contributor Sprint Retros’ a makeover
My favorite item in your list I think it could help a lot to make the retro format more attractive to read, both visually (eye candy!) and with its content (ability to get useful information out of it quickly). Maybe turn it into something that resemble more a newsletter, rather than a report?
Make collaboration easier
There is a section of the retro report that’s dedicated to asking for help, and it’s definitely one of the most used and useful I think. Building and iteration on top of this could be a good idea.
Improve ‘Sprint Retrospective and Planning’ reminders and questionnaire
+1 to what you mentioned. I would also look into improving how we notify/chase people - currently it’s completely automated, and we all tend to disregard automated emails much more than individualized ones coming from a real person. Imho we also need to allow a real person to supervise this, and allowing to talk more individually to people who haven’t answered could help. The automation/tool could focus on facilitating the work of that person: list the people who haven’t replied, allow to email them directly and easily (but with a custom message), ordering by the number of checkins missed, linking to their slack PMs to use an alternate method to contact them, etc.
Maintain a list of Core Contributor projects
This already exists - that’s the roadmap, and the tickets on the Open edX org. But maybe we can find ways to better utilize those, and make them more visible?
Make Core Contributor sprints longer
Personally I plan every two weeks, but why not. It could lower the fatigue induced by more frequent checkins – but on the other side, longer sprints are much less agile - and when we discover that we need help at the beginning of a sprint, that’s a whole month to wait until being able to post about it.
Enable Discourse Chat
Wouldn’t that duplicate the role of Slack? Though maybe an option would be to replace Slack altogether? If anything, it would allow to get rid of a proprietary software dependency, likely reduce Axim’s bill, and regroup discussions on a single tool, instead of two.
Survey
Also +1 to your suggestion done elsewhere to do a poll/survey about this, it would be useful to get the answer from all the core contributors. Imho having a way to select some of your suggestions (the ones people think would be useful), and then rank those, would be a simple way to provide that feedback. Also including an open suggestion field (“other”) would be good, to get additional ideas. And maybe with a link to this thread for further comments on the individual items?
To make sure we get everyone’s input, using a survey software which allows to track who has answered or not, based on a list of emails, would likely be useful - to allow us to keep track of who hasn’t answered, and ping them or ask them more directly, as necessary?