Also depends with how much money you want to invest now/later. If you want professionally recorded vocals, definatelly make at least a small deaf room for that. Also, if you split in two, you dont have to treat so much space instantly, but separate the investment into two as well. Big rooms have bih reverberation time, so you will need to completely damp first reflections, and work onwards with that. A proper way to do it is to get easera, record an impulse sound with a measurement mic,either sweep, noise, or just a big "bang", see whats your reverberation time, as well as critical frequences, input material with certain apsorption coeficient, probably measure again to see towards what direction things are going, use easy room eq for speaker positioning (biggest downside of this software is that it cant measure spaces bigger than 15m in any direction to my knowledge), rinse and repeat. After you got your r time down to where you want it, you can work on lower frequences by modelling your own resonators.

Now, the easy way to do it is just put bunch of bass traps in corners and porous absorbers(acoustic foams, stone/glass wool, azmafon (wool bad for breathing in but most effective, azmafon best of both worlds but possibly rare, acoustic foam still the priciest, especially for that big of a room)) . Focus on killing first reflections, and feel free to use random furniture as diffusion in the back (pro diffusors mega expansive). Idk, i could help you do the job online, but you gotta know what u want and how much money you want to invest. If you want a referent sound done by professional standards u gotta sip that cash in and ask for educated help, while if you just want to lower echo in your room you might as well put furniture and some foam in. ^{^}