Let me describe my situation as best I can.

This is a private 3 bedroom residence located on Long Island, NY.

Just had a backup of wasteline, which is below foundation, into our
unfinished basement. The main line was cleaned; papertowels were
found. However, today when doing the laundry, I saw some water backup
in the (future/rough plumbing for a) shower drain pipe, i.e., the low
point.

The property survey map shows that there is a septic tank with a Y
output piping to two overflow pits. I believe that the overflow pits
are the precast concrete type, but have not ever seen them nor do I
know their size/capacity. The location of the septic tank, which has
a standpipe from a previous service event, and the location of the
overflow pits are indicated on the property survey map document. The
overflow pits most likely do not include standpipes, as they have
never been serviced.

I am arranging for a draining of the septic tank.

The guy that did the mainline cleaning wants to install a standpipe at
one of the two overflow pits and add chemicals (35$ per 5 gallons and
he recommends 15 gallons) to that pit. Is that the way to go ?

I am thinking that a septic tank draining is the next step, and
possibly the last step needed at this time.