There are so many ways to remedy this but I will attempt to give you the easiest vs the most comprehensive solution.
As others have recommended, getting panels on your first reflection points may help with this issue. I would recommend gik acoustics or doing a DIY panel build. Not hard. Just need some stiff insulation and fabric and frame.
Another thing you can do is get a Umik-1 and use room eq wizard and see what your frequency response looks like both on var smoothing and at 1/6 smoothing. You may be able to determine where that resonant frequency is and use a notch EQ filter to reduce that band.
And if you really want to go spend some money, yes maybe it's the focals. I recommend tektons for detail without glare or harshness. Double impacts are my go to recommendation under 5k