Searing first is worth the extra pan. Pat the pork completely dry, get the pan screaming hot with a neutral oil, and brown it all over for 2–3 minutes a side. After you transfer the pork to the slow cooker, deglaze the pan with a splash of the soy sauce or rice wine — scrape up all the dark bits and pour it all in. That's where a lot of the flavour is. For a sticky glaze at the end, take the lid off for the last 20 minutes so the liquid reduces and the sugars caramelise onto the meat.
Use tamari instead of soy sauce and GF oyster sauce. Everything else is naturally GF.