What affects the reliability of bike more, better suspension or heavier tyres?

I have bought a hardtail (Rockrider ST520) with 80mm front travel and 27.5 x 2.0 all terrain tyres.

The bike is meant for slightly rugged trails but I intend on taking it out on some fairly rough ground, it will probably hold as I am only 60kg but what upgrades are possible to increase its strength and reliability, should I invest in a better fork or heavier tyres?

I'm a college student so finances are how you would expect them to be.