I would do this (please refer to sketch attached):
1. Buy a lot cheap fluffy insulation for a test. Personally I prefer polyester over fiber.
2. Stack piles of fluffy in corners behind credenzas on window wall, floor to ceiling. 20-30cm thickness in total at least.
3. Second layer of fluffy under desk, below window. You might think to put it up during session over the window to cover glass. Or could be too layers, for session extantable from floor to top of the window. You will need frame here. Perhaps 10cm+ thickness.
4. Incorporate fluffy into credenzas side on reflection points. Perhaps 10cm thickness.
5. Put as much fluffy in back wall corners, floor corners and ceiling, if possible.
Measure with mic or your ears. I would say that should reduce reverberation, flutter and perhaps some bass. Will not be idea by low cost reasonable improvement. Test it if you like it, if yes, unpack fluffy and put in some sort of framing to have a less ghetto look.
PS: Search this website for similar problems and solution, particularly in construction section. You will find more examples and details there.