I suspect your bike has poor shifting and you believe the derailleur is the culprit. This is not always the case. The shifting is a sum of many parts:
- Shift lever
- Quality of cables and housing
- Proper length of cable housing segments
- Cassette / chainrings
Not only that, but the system can be damaged. For example, if the derailleur hanger is bent you won't get any better shifting by changing the derailleur to a better one. In fact, you won't get any better shifting by changing every listed part to a better one!
I used to have a really cheap bike with Shimano Tourney derailleur too. The shifting was crap, and it stayed crap no matter how much I adjusted the derailleur cable tension barrel adjuster. The cause? A bent derailleur hanger.
To get a 100% guarantee of better shifting, you first must diagnose whether your bike has a bent derailleur hanger. Then you must replace every part in the bullet point list with a better one.
Anyway, derailleurs have a wider compatibility than what manufacturers claim. You don't always need to change a 7-speed derailleur to another 7-speed one.
From the bible: https://www.sheldonbrown.com/speeds.html
Within a given brand/style of rear derailer, all "speed numbers" are generally interchangeable. This applies to all indexable models, basically everything manufactured since the late 1980s.