A few years ago I had an authorised service agent (in Mosman, NSW) replace the battery% in my 2015 Retina Macbook for about AU$400. It was well worth it.
Although I now have a MBA M2 as my main office computer (with a 32" LG display - replacing a dying 27" iMac) I find the Retina Macbook is still brilliant for travel.
% My understanding is that the Retina Macbook battery is in several pieces distributed through the guts of the laptop. It was a way to keep the exterior dimensions as small as possible.
Update: must have jinxed myself. I just checked my partner’s 2016 Retina Macbook and its battery is only 70%. I am investigating getting it repaired.
I used an App Store app called Battery Monitor to determine the 70% value. The Apple System Preferences/Battery function just told me it needed repair. But it did take me to a relevant Apple web page for organising a repair.