Dips (and nodes) being localized is an intrinsic property of standing waves, and their frequencies and positions are exclusively defined by your room’s geometry. Absorbing requires at least 1/4 of the frequency ‘s wavelength ; in your case, it takes meters of absorbant. Plus, it only fixes peaks, not dips. Once you admit this physical reality, you understand that there an only be a compromise, which is linearizing as much as possible the respinse curve, for one seat at listening position (or averaging two or three seats), using parametric equalization. This is exactly what the commercial room correction solutions do (audyssey, dirac etc). And it is only useful below 200 hz.

You can’t fix this with any form of absorber, filling a dip would rather require selectively amplifying these frequencies, which is also unrealistic.

The only realistic approach is to use a parametric equalizer to attenuate all ther frequencies, by say 10dB to keep it realistic.