I bought the UMIK-1 in 2022 and made the first measurements, using REW filters. So far so good, I cannot describe the audible change, but something from the "unqed" sound disappeared. The motivation for today's measurement is the recent use of IEMs and comparing the sound of IEMs and loudspeakers. I want to get closer to the sound I hear from IEM. From 2022 I have a filter with 5 bands, from today's measurement I have a filter with 20 bands. So, let's say I am still in the learning process in this craft.
I am unsure if a subwoofer is an option since I live in a flat surrounded by neighbors. And no one has claimed against my hobby, that I would rather stay as a good guy.
My room measures 3.5 x 6 x 2.6. The speakers are placed symmetrically in the room with the longer wall. When I clap my hands I hear a reverb, so the damping could be better.
Don't bother clapping your hands as a test for reverb. That only tests the reverb in a narrow band of frequencies. It's for those plebs who don't own microphones
You have a UMIK-1 and REW, you can do much better. In fact, you already have the measurements, all you need to do is look at it. Click on the RT60 tab, Energy-Time Curve, spectrogram, and waterfall. That will tell you much more than a hand clap would.
Re: subs. Yes, annoying your neighbours can be an issue. When I lived in an apartment and bought my first stereo, I spoke to my neighbours and told them that I want to test how loud my speakers will go before they can hear them, so that I can be a considerate neighbour and avoid disturbing their peace. The idea was received very well. I picked an evening on a weekend, and gave them my phone number. I asked them to ring me when they could hear my music. Then I turned up the volume every few minutes and waited for the phone call. It turned out that I could actually turn it up pretty loud.
It helped that I was using some strategies to avoid noise transmission. The biggie is sealing as many air gaps as you can. Close the windows and throw clothes against the door frame (a lot of sound leaks out the door from the gaps, especially at the top and bottom of the door). Consider installing self-adhesive foam gaskets on your door frame.