I really wouldn't recommend it. You've had great changes just mewing so that's brilliant. slow pressure always wins. If you expand rapidly with the expander you risk tipping your teeth and worst case scenario detaching your upper jaw completely from your palate so you will widen your palate but not your upper jaw and trust me you dont want that Flat cheekbones wide palate no no no. Your getting good noticeable change why risk that to mess your oral posture and your beautiful teeth. Palatal expanders are for people with narrow palates who cant fit their tongue their and have trouble with getting that oral posture, you've got it already! you dont need anything else. If you try to maintain that tongue to palate posture throughout the day as much as possible you'll naturally do it at night. Also a bit of mouth tape on your lips at night wont hurt.
Well, actually, when I do mew the edges of my tongue do touch the teeth quite a lot... not sure if that's normal however, but the tongue doesn't fit neatly and perfectly like I feel it should do. My jaw is quite wide and defined actually, I chewed a lot of gum as a kid so I think that's why (and I still do, that being said my jaw is lopsided due to teeth grinding on my left side of the face). I'd like to widen my face since my face is way too thin, I'm quite young so results should definitely be possible.
I also use mouth tape on my lips every night, so far so good.