Oxalic acid has been considered a good flush for more than 50 years.

https://www.astm.org/stp25166s.html

The evaporust special gell certainly looks interesting,
maybe after a first oxalic acid cleaning?

A video boroscope “snake camera” to view interior passages after a flush seems logical,
since their price has come down so much,
and they have so many uses.
The smaller than 9 mm camera diameter is very handy.