Starting acoustic treatment in my room

Hi! Here is my first post in this forum.

I'm looking for advice and help to understand if I'm taking the right steps to acoustically treat my room.

The tools i'm using are amroc THE Room Mode Calculator and acoustics modelling website.

My intentions: start treating my room acoustically to solve with little budget to have the best result. Over time, I will improve it piece by piece.

My room: 500x330x265cm

I'll start with the 38% room rule.

The material I choose are:

-8x WLG 045: 3000 Pa·s/m² Caruso Iso Bond

-4 to choose for sides

-Wood panel (on research gate i found info about wood) with around 120000 to 150000 Pa·s/m² (i can have this for free, so i had an idea that i will explain)

Since aiming at low frequencies requires a lot of material, and a lot of material means a greater expense. I would like to start the discussion starting from 90/100 hz, a starting point that I found adequate to take care of some room modes, and treat the points closest to my listening point

My idea is to start with treating the front celling and the front wall with the 3000 Pa·s/m² Caruso Iso Bond, 20 cm of material + 20 of airgap.

Front to back wall major issues: 68 hz (Absorption Coefficient only around 0.5), then 102,9hz with 0.8 Absorption Coefficient and 155 hz.

Back wall major issues: 63 hz (the absorber will have only around 0.5 of absorption), then 127 hz (here will be a lot more effective, from 100 hz the absorber is 0,8).

Now I would like to understand how to proceed with the side walls (I would like to position a panel on the left and one on the right, without being excessively invasive with the thickness in this case. The problem is the room mode at 52 Hz and especially the one at almost 104 Hz, which is very close to another room mode. The initial idea was to use WLG 035: 10,000 Pa·s/m² of caurso isobond and maintain a thickness of maximum 20 cm (10 + 10). The efficiency at this thickness is very efficient at 100 Hz (with an absorption coefficient of 0.5. Is that enough?

In the back part of the wall I have some ''suspended'' stairs. Space 80 cm (given by the width of the stairs). My idea is to use a thin 5mm wood panel and use it as a basstrap (on research gate I saw that some types of wood have 120000 up to 200000 Pa·s/m²). The result on absorption gives me a strange (obvious) comb filtering behavior. Which in my opinion would be better to use than not, especially to help with the attenuation at 67/68 Hz (a good 0.8 coefficient!) and that at 102/103 Hz (almost 1 coefficient!). It will be a 5 mm panel with a size of approximately 80 x 120 cm I think. Is this a good idea?

I have another question, would it be better to place the front panels close and in the center of the front wall? Or one behind each studio monitor?

I also foresee the advice of treating the corners of the front wall room, but as I said, I would like to solve the biggest problems first with the low budget I have.

Notes regarding the room:
On the right side I have a radiator and a French window to the right of the front wall.

Also, does anyone know, does don-audio.com offer discounts on Black Friday?

Thanks for the help!