Usually lower end skates don’t have bearings that you can clean. They’re a sealed unit and you would just replace them once they wear out. Better bearings would definitely help... it’s up to you whether it is worth it and how much you want to spend.
And yes there is a large difference in skate quality. For starters the boot and outsole will be made of cheaper more flexible materials so that may be the flexing you’re feeling. Also, lower end roller skates usually have a two piece steel chassis, which is weaker and can allow more flexing. The mid and higher end inlines will feature a one piece aluminum or magnesium chassis.