Hey Rein,
Firstly, yes, I do have the ESPE01 module - it does change some things a little bit with respect to tuning and playing, but I spent my first 7 months playing without it. I also have access to an EWStandard without the pitch extension module, so I have a reasonable understanding of both sides of it.
I personally (key word!) don't recommend you tune to behind you. Your body position influences the linearity of your instrument greatly near or beyond 0 beat, and it makes it hard to access the lowest or lower parts of your range without having to lean - which isn't advisable. Some thereminists lean into the high register for linearity purposes (including me, sometimes) but that's far more controlled than doing it by way of tuning behind you. Personally, I tune such that, with my body just barely behind 0 beat - pretty close to the cusp of my low register - I achieve an octave from closed to open fist. To tune consistently, I recommend a few things to consider:
1. Always stand at the SAME PLACE relative to your pitch field each time you adjust it, and THEN test for tuning. In my case, every time I change the pitch field tuning, I always move my whole body so that I'm once again near to the edge of the 0 beat, and then I try for tuning. That way, your body isn't influencing how the tuning feels, which is important.
2. Make sure you tune in the same general pitch range each time. For me, since the ESPE01 module has pretty linear tuning in the middle 2-3 octaves, that's my tuning range any time I want to test my tuning, so I'll try to do an octave there. As a side note, don't worry if a 'comfortable' position for you isn't middle C. In fact, tuning to put it there likely won't be too consistent, as what is comfortable for you will change. Don't be afraid to be flexible!
3. Underappreciated step: make sure that, on extending your fingers, they are pointed more or less directly at your pitch rod. I can't tell you how much this matters. Sometimes your tuning is right, but your body is positioned in such a way that you aren't able to extend your fingers in quite the right way, or it could even be just your wrist or arm position at the time (though whole body angle is most common for me) - and in that condition you'll never be able to tune right.
After figuring out these steps through trial and error, my tuning has never felt so consistent. It completely changed my world, so I hope it helps you too. Consistent tuning = consistent playing.
As for everything else you said, it really all comes down to practice - including the vib in different positions and using different techniques. Try stretching your upper body before you play, including hips, shoulders, all parts of your arm, wrist, and even fingers. For me, it was very hard to hold consistent notes or vib in extended positions for a while, but I found that as I got more flexible it got easier, and that comes with responsible stretching and time. Keep at it, and eventually, like athletes, your hamstrings (I believe…) will stretch to allow you to do what you want to do. Again, for higher stuff in general, I recommend rotating from your hips to bring your shoulder closer to the antenna. It'll make it a straighter shot to the antenna, and will likely help clean up your pitches and your vib (and will also expand the range between notes, as it shifts your body closer in). For vibbing in general with different pitch arm techniques, do take the time to practice so you can engage those muscle groups, and they'll treat you nicer for it.
(As an aside, if you notice, Grégoire actually changes how his forearm is positioned based on the range he's playing in. As he gets lower in his range, his forearm gradually lifts up to tuck his hand in toward his body (in a natural way). That's helped me to navigate the issue you have with bass register playing.)
Again, I'm by no means an expert, but I hope this stuff is valuable to you or anyone else in some way. If you have any other questions, please let us know! Good luck!
Colin
