Pressure fluctuates much more than temperature in general. There is nothing bad about it for as long as it oscillates around the equilibrium value. Your system is rather small and that leads to even larger fluctuations. I have already explained that this in combination with the low compressibility of your system leads to resonance when you run fix press/berendsen with too strong coupling.
The only way to reduce pressure fluctuations is to enlarge the system.
To monitor the pressure it is generally better to use a windowed average.
If you want a dissipative thermostat you could combine fix langevin with fix nph.
The topic of what is an acceptable thermostat is an almost religious topic and thus I don’t want to state an opinion about my personal preferences.
Axel