I couldn't agree more with Tom's answer. I think practice itself underlies some kind of evolutionary process. And if you feel the urge to change or adjust your practice it may be worth a try.
You don't sound like you have a lack of patience or stamina, Hugh. It would be otherwise if you were changing your meditation for the fifth time in a row within a year. And you seem to think it through and try to see it from different angles (as your question here suggests). I think that are good signs. Another good sign for me would be if you're wish to change something comes with a great increase in motivation and after you've given the old techniqe a fair amount of time to keep up with that. Plus the method you think about changing to has a very good reputation and will probably help you to make progress and measure it accordingly (Daniel's book is great for that). And as Tom rightly said, your experience and your old method won't get lost. You can always get back to it or add something to the mix.
In the last 10 years I fundamentally changed my method twice, but after arriving at breath meditation it were mostly changes of my inner outlook that seemed to need some adjustments. Sometimes I needed a kick in the butt (which at the time came from this
Ayya Khema video) and another time I needed to let go of the force I applied to my meditation. So my advice would be to let yourself inspire from different teachings but also investigate what exactly the blockages you experience are all about. Do you try to get away from negative feelings all the time and are a pleasure seeker (my problem sometimes) or does your wish to change happen after some deep thinking and investigation on how to tackle these blockages? Your statement that your current practice "is a kind of artificial 'retreat' from reality" may be an indication for the latter. You seem to look for something that not only gives you bliss in meditation but makes you more content even when in touch with the rest of reality, or as you put it 'happiness independent of conditions'. That would be a good sign, IMO.
Could you describe what exactly draws you to the noting style practice? What have you read or heard about it, that makes you think it's suitable for you?