Hi,
It's great that so many people are trying to help here.
As you post Tjapukai, your practice and experience get clearer.
It seems from your last post, that your feelings about the success, or point to your practice, your perceived inability to get into the emptiness, and your busy mind, are higher on the list than the back discomfort. Perhaps that, being frosting on the cake!
May I suggest another approach, within the realm of SFQ practice.
-'Starting over' from the beginning. There is you, and there is SFQ. The practice brings benefits. This is a fact. They may be slow coming or fast, cyclic, bring great changes, bring plateaus, etc.. Inherently, it is good. So , don't place a negative message in the practice.
- SFQ is message healing, or signal healing. This goes beyond physical healing applications. The message is what you charge something with. It sets manifestation in motion. It manifests, quickly, with confidence and a focused intention. Put a positive message back in your practice.
-Engage with SFQ in a way that allows you to 'enjoy' and 'relax'. More like nap time, or a warm bath. A time of rejuvenation. A positive break. Comfortable.
-Now, to get your experience in this mode, change your position if necessary, and the time. Build it up, slowly. Start with 10 minutes. Maybe laying down, doing small universe. You are just going to lay down, relax, and focus your mind on this one exercise for ten minutes. No pressure, no long session. A time to relax. Find a position(laying or sitting) that you are comfortable in. Really comfortable. Allow yourself to really relax. Set a doable goal, like ten minutes of small universe, comfortable position, "focused mind". (smile too!)
I wouldn't look at mind activity as negative in any way. Don't react to your mind's activity. Strike a healthy working relationship. Engage with the master's energy to help you initiate and/or define this. It is good, to give the mind a rest. I find the mind receptive to this. I also find it receptive to exercising focus. It doesn't have to be perfect. You work together, to slowly build something. Some relaxation, and an exercise in focus, for a brief time. Set a doable goal. It can be minutes, regarding the mind. I would prepare and focus, the day before. Start with the message. " I am going to lay down and relax tomorrow, and do small universe for ten minutes. I will focus my mind on the exercise. I will relax deeper than I ever have, and start my practice on the right foot. I will build on this each week." Something like that. Again, ask your master's energy for help with this message. It can , of course , be briefer or differ from this. I just want to give you an idea. Focus it towards the next day. Stay aware of the goal, set the stage, keep the agreement. Be positive during this time. This is part of the work/focus. Afterwards, reflect on how you did, if you relaxed, did you have some moments of your mind just focusing on the exercise, etc. Were you comfortable...

I hope you give this a try. Short , and sweet, and building on that.
Your practice is not in a good place, where it is at. Earlier guidance to push longer, would still be a valid answer, but there are many, many other choices too. Quality, is much more important than quantity. This should be a positive experience for you. Experiencing relaxation at deeper levels will help you with the emptiness part. There is 'no' hurry to achieve this. No pressure. No failure. The fact that you practice, is the bottom line good thing. You are making the effort to improve. This is the immediate, and important, success. You are doing the work. How fast or how high is secondary, to this great intention. This intention is postive, and is healing energy that effects the whole. Working the physical part of the exercise , sometimes keeps you in that. That is okay too. With the right attitude and redefining your practice, you can make this work for you. There isn't a right way, or a wrong way. Within the practice, there is good , better, and best. Best, can mean, what is best for you , to allow you to start this out on the right foot, and begin to gain from the practice.
If you want to, you can consciously release thoughts you have or had. Look at it all as energy. If a thought doesn't support you, isn't good for you, send it back to the universe as extra energy you don't need. There may seem to be an endless stream of thought, but it isn't. Again, don't react, just release. Or, if focusing the mind, just do that, with no other thoughts. If one comes, simply go back to the focus, again, without reaction.
love,
gallen