The microphone is THE interface between your voice or an instrument and your audience. Better microphones offer more presence and clarity and more stable feedback protection when used on stage.
Old cables become brittle at the usual stress points over time and the outer protective sheath may already have suffered some damage. Investing in stable and durable cables with good shielding is always underestimated.
A better mixer has great potential for better sound: microphone preamps are becoming more and more sophisticated and offer less distortion and less background noise, while integrated effects significantly enhance the signal. With tablet mixers, the control surface is decoupled from the mixer so that the sound engineer can move freely in the audience area during the sound check.
Even older loudspeakers can still deliver pure volume. Many developments in cone material, cabinet design and amplifier technology in recent years have resulted in a better presentation across the entire frequency range, lighter speakers thanks to new amplifier models and a wide dispersion into the room.
Buy a sub as well. A good bass improves your sound tremendously.
Don't leave the low frequencies exclusively to your normal PA speakers. A dedicated speaker for the bass relieves the strain on your main speakers, which can now take responsibility for the low-mid frequencies. As bass frequencies are distributed relatively omnidirectionally in the room, the location of a sub-speaker is only a minor matter.
