Yes u need to change the cust.prop in OP. Change the target region country to US (from KR) and same for target country. Change language prop to en_US.
Additionally, change featureset prop to OPENUS from OPENKR.
these changes disable most KR elements in firmware, most noticeably dual clock and call recording in dialer. They **may** also enable volte if supported by carrier.
This is from nice guy Neo on tele group who's played with this much more than I have
Oh yeah, lol, just remembered, the main reason I was asking for the info above was to get the 'field test' selection (so you can select LTE bands) in the SVC Menu from dialer. By default, it doesn't show up, but above changes fixes that also.
did you still do this on your last flash? would you mind sharing every modification you made