IMO, matching pH in new salt water is not needed or useful, and adding soda ash will boost alkalinity. IO already has plenty of alk, too much for many people.
From two different batches of Instant Ocean Sea Salt (purple), once from the bucket, once from the box with 4 bags, I've had wild results. Yes, on average, the Alk was high. Always with 1ppm RODI, mixed with air stone & heater a full day beforehand.
FWIW, I often have a brown scum floating on top, I scoop it out.
Once, the new SW (from the bottom of the bucket) had calcium off the scale, over 500ppm. This last time, from a bag, I didn't test anything. I have a PH meter on order.
I did the manual test for PH in the DT and it was lower than expected, so assuming it came from the 50% WC.
Even my chaeto slowed growth dramatically.
Lesson learned, I will always test my new SW and refrain from doing big WCs, now that the tank is well cycled. Too bad, it's easier for me to do 50g than to do 20g.