Set the 5 gig limit to each MS device you have
I just backed up my phone as per recommendation (1.8gig)
Right now you do store credits and office 365 free for a year as a free add on. (onedrive?)
I understand you need to reduce this but some of us have 3 devices and still only get 5gig. I use 7 gig total which is not much for 3 windows devices and is my hub for cross device file sharing. a single device not so much is necessarily needed to work..
Anyway I think 5 gig per device in your account would be reasonable allotment and reflect the added base storage used by owning multiple devices,.