Looking for info on replacing MTB derailleur
Looking at the Minelli web site it seems their bikes are fairly inexpensive, using Acera, Altus and Alivio groupsets.

The latest Acera, Altus and Alivio groupsets are all 9 speed so I have to assume you have a slightly older bike when those groups came in 7 speeds, Or you might have a Tourney derailleur I guess.

Basically, you will just have to find a compatible 7 speed replacement derailleur. I'm afraid you don't have much choice in terms of 'sturdiness' as 7 speed drive-trains are now the lowest level, cheapest offerings. The good news is that a replacement will be inexpensive.

If you want to upgrade the bike, don't do it a component at a time, just buy a new or used better bike. There are plenty of answers to questions on this site that show how drivetrain component upgrades inevitably require replacing most or all of the drivetrain, and sometimes wheels, which is very definitely not cost effective.