A common problem is that your stage volume is way too high. It's not good if you can't hear your own sax! But if you turn down the amps, then you'll need to feed more of the amps through the mixer for the mains. The keyboards and the bass can go direct using 1/4” cables, but the guitars will need to be miked. Once this is done, then you can face all of the amps in toward yourselves so you can hear each other even at lower levels.
With this setup, then you should be able to get by with a simple monitor mix for the vocals. I might suggest using side-fills instead of putting the monitors on the floor. You could put a third one behind the drummer so he can hear too. Remember that the sax and guitar leads should be treated as vocals and fed into the monitors when needed.
You'll need a mixer with at least 12 channels, but your existing PA amps and speakers should be enough - you're not getting louder! The only possible issue here is that you may be putting more of the bass guitar through the PA, so you may need an extra sub woofer and amp, but maybe not.
The last issue is to make sure you have an EQ for the monitor mix. This will help prevent feedback, and make the sound much clearer.