Yea, I feel the same. It just made me want more.
But I've red something interesting on reddit:
https://www.reddit.com/r/musicproduction/comments/zju7c0/_/izy7wyu
I've checked with my room dimensions and 3,5 meters is around 49 Hz (7 meters wavelength / 2). If we count in the niche with a wardrobe it goes down to 43 Hz (8 meters / 2) and gets pretty close to my measurements.
From the text it seems that Mr. Hidley is considering acoustic treatment and room design only; with these means it can indeed be very difficult (and expensive) to address the low bass but they can improve the sound in the whole room and regardless of source.
However bumps in low-frequency response at a single listening position can very effectively be removed by PEQ. This doesn't fix the room (i.e. any other sources or locations might still sound bad), it just adapts the audio system to the room and its position in it.
While I haven't tried to reach 20Hz reproduction in tiny rooms, I personally can't see why that couldn't be done with appropriate subwoofer choice, integration and DSP PEQ.
Remember also that we can reproduce low bass with headphones within the tiny space or their earcups (or within the ear channel itself, if speaking of IEMs).
As I read them, your in-room measurements suggest that the subwoofer works fine. You seem to be getting sub-30Hz response at some points in the room. Note that the sub goes to 27Hz at -10dB anechoically, so I wouldn't expect it to give you flat in-room response down to that frequency.
If it was my setup, I'd put the microphone at the listening position and I'd try to move the sub around a bit and do sweep measurements for each position, to see if I can get a nicer-looking response in some positions. Note that it is better to choose a position with bass bumps than one with severe dips - the bumps give you headroom and can easily be knocked down with PEQ later.