As you have found, the upper frequencies are easier to manage even with aesthetic decor. You could ultimately address initial reflection points with panels to help with reverberation. Tips: Absorptive material is more efficient when used on all surfaces rather than loading up on just side walls or whatever. Also the material should be as thick as 1/4 the wavelength to be addressed.
The lower frequencies are going to be a challenge. Wall (porous) panels are not going to help with room modes. You could employ broader band panel absorbers. And you may be able to get an idea of modes using online bobgolds calculator, but unlikely its going to be wholly accurate.
If you are able to place speakers using golden ratio, that may be a better staring point. The issue is the golden ratio placement can be tough in small rooms. You may have to finagle it some. Yours speakers are placed at distance of a multiple two which is not likely a good starting point.
The lower frequencies are going to be a challenge. Wall (porous) panels are not going to help with room modes. You could employ broader band panel absorbers. And you may be able to get an idea of modes using online bobgolds calculator, but unlikely its going to be wholly accurate.
If you are able to place speakers using golden ratio, that may be a better staring point. The issue is the golden ratio placement can be tough in small rooms. You may have to finagle it some. Yours speakers are placed at distance of a multiple two which is not likely a good starting point.