First thing, instead of converting the SLC to GRDH, the last mode is most probably available for your AOI, In this case you have the GRDH, you could subset your AOI directly,
But in case you’d stick on this method for some reason,
Slice assembly operator is design to merge the two subswaths belong to the neighbor images, However, merge operator is the right one for merging the subswaths within the same image.
The steps of converting SLC to GRDH
Please take a look at this post,
or directly SLC to GRD, take a look at this post,