Ok so I have a 2012 giant sedona. My gf is looking for a better bike and people keep saying the box stores have ****ty components....well I got looking and my bike has the same....did I get hosed....xan I upgrafe the cassett/derailer etc.. should/do I need to?

Components : shifter sram twist, fromt derailer shimano m191" rear derailer shimano tourney, cassette Shimano TZ31 14x34, 7-Speed, crankset SR Suntour XCC 3-Piece, 28/38/48

Now this bike is my only one I'm 365lbs and a recreational rider but do wanna do a metric century next yeat and maybe a few 50mile rides....

I don't know anything boutbikes so any help is good