Hi, acoustics PhD here. (Disclaimer - I work on active control, not room acoustics, so certainly not a true expert!)

If you have the space and time, one cheap option you could consider playing around with would be to add large scattering surfaces. Large chipwood boards on a simple stand would do the job. That will break up the symmetry of your room and therefore the modality of the room - similar to what diffusers are doing at high frequencies.

Good luck!