Upgrade materials create the most value when they strengthen equipment you use consistently. Test first, then commit resources.
Keep one main weapon competitive
A reliable main weapon gives the build a stable source of damage while you explore other options cheaply.
Test movesets before investing heavily
Use a weapon enough to understand its range, timing and role before spending scarce materials on it.
Upgrade when damage is the actual bottleneck
If positioning or survivability is the main problem, more weapon levels may not solve the encounter.
Switch when a new weapon has a clear role
An alternative is worth investment when it meaningfully improves the way you want to play.