I'm no expert but a few points that I don't see raised that the contractors
who empty mine made:

Septic tanks are/should be sized to the house so there is the balance
between number of users/size of tank/frequency of emptying.

The "bottle" tanks with the floating ball can be knackered before the
soakaway gets damaged. The solids accumulate under the ball and it can
become impossible to depress the ball to get even a small suction pipe
into the tank.

The undigested solids settle at the bottom of the tank and if left too long
will not be sucked up and unless the operator goes out of his way to
properly empty the tank it starts to loose capacity and in effect becomes
too small for the house. (no matter how big the pump is it's sucking and
1bar is all you can get)

PeterK