Hi rixthetrick,
Difficult to apportion a budget as a percentage of total value of your system. I think of it as a case of taking X amount of $ to tame the room's acoustics which then will provide the correct acoustical environment to allow any and all systems to be heard at their optimum level of performance.
The room proportions do make a difference but are seldom found in the real world. The golden ratio or Fibonacci Sequence help with spreading the room modes so you do not get massive build up of certain frequencies. This is seldom found and is not an insurmountable problem. If possible avoid any one dimension being exactly twice or half that of another. So a cube would be absolutely worst case.
Any room, however, can be made to sound good. I made all my bass traps, diffusers and absorbers myself which does not cost that much. I used to visit demolishers and buy the insulation panels for peanuts. Look for Owens Corning 703 or similar.
Start with bass traps which need to be rather large but the divorce is worth it for good sound. 😎 These need to be big, you're dealing with long wavelengths so those dinky little scraps of foam Amazon sells are useless. Google superchunk DIY bass traps for an idea.
Corners are the place for BTs, any corners, the more the better. Usually the 2 front vertical corners are used for floor to ceiling traps but instating them horizontally along 1 or 2 wall/ceiling corners work just as well. This alone will make a very large improvement.
Place absorbers at the first reflection points. As I mentioned curtains, drapes or skimpy wall hangings do not absorb over a wide range of frequencies. Build or buy frames at least 4" deep to act as broad-band absorbers. The OC703 is available in panels 2'x4' so use frames to size or multiples. Panels 4'x6' or 3 panels 4'x2' with a small space between look smart and can be covered in different colour fabric.
A 'cloud' attached to the ceiling is a great way of eliminating the troublesome floor/ceiling bounce and does away for the need of a carpet or rug, though a rug will not harm anything. i helped a mate with a ceiling cloud and fitted an LED strip light around the perimeter on top. With the remote that came with it looked very neat, could change colour and dim to suite.
I am selling the old stone cottage and have abandoned any further work on the acoustics. I use Omnimic to measure which shows a consistently good T60 but also shows a slightly lumpy bass. Based on before, during and after screens that I need one more BT and job done.
Persevere with what you have started. You are clearly DIY capable so go for it and good luck.