Agree with most everything already stated here. My story is an illustration of how challenging this is. I live in a rural area on a two acre lot on the edge of a cornfield. My closest neighbor’s house is more than 300 feet away on the other side of the road. I built a pole barn several years ago and decided to build a small studio space as part of the project. The interior size of the room is 16 x 20. The walls are 6” thick, the interior wall framing is separate from the exterior wall and the void space is heavily insulated. The interior walls are double layer drywall. The ceiling is framed like the walls and again heavily insulated. The room is well treated with acoustic panels, corner traps and two large bass traps. The floor is commercial carpet on a heavy rubber pad on concrete. There are two small windows for some natural light. There is a separate heat pump/AC unit for climate control. The door into the studio from the interior of the building is an insulated steel exterior door. I didn’t account for it separately, but the build cost of this space was between $12k and $15k.
When my band started rehearsing in the studio, I wanted to see how loud it would be for my neighbors. I had one of the guys play my bass on a few tunes while I went outside for a walk. At 50’ away, I could hear most everybody, though it wasn’t loud. From my neighbor’s front porch, I could easily make out the entire bass line, the kick drum, and most of the snare hits. The SPL in the middle of the room was about 90dB, with transient peaks just over 100dB - pretty much our live performance volume.
I’m fortunate that my neighbors are cool and like the band. Even so, we’re rehearsing with IEMs, low amp levels, and we’re always done by 10PM.
As others have noted, you’re not going to fix your space, so its time to explore other options. Good luck - Let us know what you end up doing...