I'd recommend dolomite, which can be bought at any garden shop. MgCa(CO3)2
it doesn't add a lot of sulfate or Cl to the system, targets just what you
need. Unless finely powdered it won't bring up the gH really fast, but it
will work.

This is good too.

Why is SO4 bad though? Cl is a given to a certain degree but SO4?
I've had very high levels and never saw anything over a number of years.
Regards,
Tom Barr