Toyota IQ service overdue

Hi!
Recently my friend bought small city car Toyota
(Scion) IQ 1.0 petrol 68hp manual 2012 50k miles
on it.
I was doing her a favor and changed oil and filters
on car. I have noticed that old oil was very dark,
thick and smelled burnt. Also oil level on dipstick
was on minimum. Air and AC filters were also
pretty cloged up so, by looking into service records
it looks like car hasnt been serviced for last 20k
miles and almost 4 years. In old oil I could notice
small metal flakes and after few weeks, when I
checked oil level for her and wiped dipstick, there
were also tiny metal flakes in new oil. What are
your recomendations? Just keep regular
maintenance until it inevitably dies, oil flush, or
something else?
Thank you in advance!