Best price/performance compromise in acoustic reflections treatment-Acoustics Forum-Acoustic Fields Forum

I am finishing a small recording “studio” for my son, built in something like two garages interconnected, both consisting of bricks walls and concrete pillars, in each a floating room built with a sadwich of three 1″ medium density panels flooring resting on hi density 3″ rockwool + lead-rubber isolating sheet in between, same for the walls and ceiling, but chalk drywall (3 panels thick) instead of MD panels. The same goes for the control room but only two panels thick and with no lead-rubber sheets in between. The two rooms interface each other via two double glass isolated window + two 40 db each isolated doors. Rooms don’t have any point of contact between them. Space between the inner “boxes” and the outer original brick/concrete structure (ca. 4″) is filled with 4″ ordinary rockwool not pressed, Entrance is into the control room via a 200 kg 40 db “isolated” security door. I finished our money. What cheapest but still working solutions might I apply in order to treat the interior reflections etc decently? Thanks for any advice.
Daniele, Italy