Visualization produces theta waves.

The hypnogogic state also produces theta waves.

I do these relaxation exercises: ( http://ncu9nc.blogspot.com/2020/08/preparing-for-meditation-with.html ),  which include visualizations, to prepare for meditation, but I don't go all the way into the hypnogogic state, by keeping the mind busy with the relaxation exercises you can stay mindful.

Actually at a certain stage I let the mind wander until I feel myself make a transition and then I go back to being mindful. After the transition there is an afterglow, the jhanas are accessible, and I don't feel any unpleasant emotions. Sometimes if I am experiencing a lot of emotional turmoil or stress I go into this state just to get a break from it all. Otherwise, in normal situations I can keep the state going after I finish the exercises if I stay mindful.

The transition can occur while you are being mindful, but very often it would happen just as my mind wandered for a moment, so I made "letting the mind wander" part of the technique at a certain stage.

My best guess is that brain waves are synchrionized to processing that happens in brain so if you had much longer pathways in which signals take longer to get from wherever they start to consciousness then it would make brainwaves slower and you more conscious even when your brain had longer brainwaves.

I have not tested this theory because I have no measuring apparatus.
This topic however is from the ones which I can only call as "very interresting" and thus worth investigating. Thanks for suggestion

What interrest me also is why you want to be more aware with longer brainwaves?
Wouldn't it make it much harder to fall asleep if you could sustain awareness in long delta waves?

The "trick" is to maintain the concentration and self-awareness, associated mainly - in simple termes - with the gamma activity. For example, during lucid dreaming, gamma activity (40 Hz) is enhanced [1]. Meditation practice (Samatha and Vipassana) will train this ability.

With regard to Vipassana, the mindfullness state during this type of meditation is "associated with a decrease in frontal delta (1–4 Hz) power, especially pronounced in those participants not reporting drowsiness during meditation. Relative increase in frontal theta (4–8 Hz) power was observed during meditation, as well as significantly increased parieto-occipital gamma (35–45 Hz) power, but no other state effects were found for the theta (4–8 Hz), alpha (8–12 Hz), or beta (12–25 Hz)" [2]. Also here, increased gamma seems to play an important role.

[1] Voss et al. (2009). Lucid Dreaming: A State of Consciousness with Features of Both Waking and Non-Lucid Dreaming. Sleep, 32(9)
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2737577/
[2] Cahn et al. (2010). Occipital gamma activation during Vipassana meditation. Cogn. PRocess, 11 (39), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2812711/