Well, I need to first dispute your 1st step.  In the modern age and for home theater, the room size has proven to be of reduced importance for smooth bass.  While I still recommend a rectangular room for the best results, the use of proper subwoofer placement techniques and equalization has made it possible to get smooth bass response in any room.

I also will say that bass traps can have a positive effect on response and, if properly used, can be quite beneficial.  The problem is that most "bass traps" absorb very little below 80 Hz and are not useful against the strongest room modes.  Those that do absorb lower frequencies tend to be quite large and impractical.  I believe the best approach is use your steps 2, 3, and 5 for the smoothest sub response.  I plan on writing an article on the concept soon.

There are some companies they said they have bass traps that can absorb up to 40 hertz but I never tried them.

I am looking forward to read your article. Also, if you can mention where to put them I know they treat the 4 corners because the energy stronger their but there are actually 12 corners in the stander room these must be addressed as well.

Thank you Gerry,

The placement of bass traps depends on how they absorb sound energy.  A diaphragmatic trap absorbs pressure and a kinetic absorber absorbs by slowing down the air molecules.  A thick fiberglass panel is an example of a kinetic absorber (see my article Acoustical Treatment Primer: Absorption on this site https://homeacoustics.org/2017/04/acoustical-treatment-primer-absorption/).  The diaphragmatic traps work best in the corner where sound pressure is strongest.  Kinetic bass traps are less useful in the corners.

The diaphragmatic traps can be built to absorb lower frequencies and while not as large as a kinetic version, they are still quite imposing.  The most popular versions do not work well at the low frequencies where the worst modal energy lives.  Also, the strongest modes are often below 40 Hz.