2011 outback lights on

So after spending $230 to turn off the lights
(brake, skid, check engine, Cruise) I was told it
needed a new catalytic convertor & brake
sensor costing approximately  $2000! After a
quick computer check the tech said it may not
be the brake sensor due to a battery voltage
problem! The dash lights are on 1 day off the
next! I also find it interesting that when the
dash lights are off it acts like there is a belt
slipping & makes a little squeal but when the
dash lights are back on it stops. It continues
to go back and forth! HELP!!!!
I’m reluctant to spend the money on it after
reading all of this. I feel like the answers I got
at the dealership were wishy washy