Hi PapaNoz, and thanks for your excellent post/question. Please know you are not alone with your concern as others in the community have expressed similar issues when it comes to the 'marks' and 'redness' left by the cannula tubing, which appear to affect some patients.
There are several remedies you can try - some are more 'ad lib' than scientific. You can evaluate the suggestions personally, to see if any appeal to you with any level of practicality.
First, this may be related to the brand of cannula you are using. Some brands are stiffer than others. Although we do not recommend or discount any particular brand, the Salter name, historically, has been associated with a softer-style tubing.
Second, some members have shared that they position or brace a 'cushion' (gauze, cotton, etc.), as a buffer between the cannula tubing and the skin.
Finally, although this is not by manufacturer design, some patients position the hose over the ears, and behind their head, rather than over the ears and under the chin. As mentioned, this is not the way the manufacturers intend them to be used, but I have seen it done with success in some cases.
I'm hopeful others will see your question and post their professional and/or anecdotal experiences here for all of us to read.
Wishing you well,
Leon (site moderator COPD.net)