Hello everybody

I have a city bike (tourney) 48-38-28, cassette 14-28 7 speed (total 21 speed) (14-16-18-20-22-24-28)
When I am riding on flat road (not climbing or down my english not enough sorry) my muscles is tired at gear 2-3 (front 2,rear 3 38-22) (if you will say that you don't have condition,I ride bicycle 200 kilometers every month)

if I want to change casette (12-14-16-18-21-24-28-32 ) or buy a new bike (I am thinking buy a new bike for a lot of season)
​​​​​
2-3
my bike 38-22
new bike or new casette 38-24

​​​​​Can you compare around? is it worth?