Great that you are working on something like this @recruit. I did a project like this for my family and friends in 2015-ish? When I got started we had a mix of devices Misfit, Fitbit, Moov, Garmin... and of course a host of applications that came with them. Creating one website that displayed everyone was seemingly impossible. What I ended up doing was porting all of the data to associated Google accounts using Google Drive and Sheets then pulling the website data from there. That enabled me to normalize the data shown for everyone regardless of device. I found ifttt.com great for that.
That was the good news. The bad news is for whatever reason you can find that most devices have some kind of data pushes on IFTTT but for some reason I have not seen recipes for MyFitnessPal on there? As recently as August I have seen the MFP community complaining about that Another MFP user seeking IFTTT integration MFP have been alluding to some kind of thing for awhile but I haven't actually seen anything. Before I wrote this reply I looked around a bit and all I was able to find was the complaints about there not being something done and not a whole bunch of API integrations. As far as I am aware the MyFitnessPal API is private-access only? I hope some of that helps? Good luck with your project.
roy darling *my posts seem a lot shorter in my head