fast battery drain of iphone 6s plus, works on 1% for hours

my 6s plus is a bit older than 2 years. recently i see problems with its battery, it drains fast and it seems to charge too fast. battery setting says maximal capacity is still 87%, so okay.

if you look at the screen shot: i charged to 100%. without doing much, battery drained to 24% within a few hours. i made a reset. after, battery read 80% again for a short time, then dropped to 24% and soon to 1%. i left it switched on over night. in the morning, i started using it again, on 1% - quite some activity, actually. as of now, it is still working. hours of standby and 2 hours of use on 1%!

any ideas what is happening here? how can i get normal readings back?

thank you!

(and yes, i know there is genius bar)

iphone 6s plus 64gb ios 13.3.1