I understand what you are saying but despite extensive search, I didn’t find specific answers to my questions.
Is there an official commitment regarding Sailfish OS support in general and on the J2 specifically?
As said, I only found the commitment for the Jolla C2.
My bad, written on the Jolla Phone Pre-order page but I scrolled directly to the Tech Specs and missed this box: Long-term OS support, guaranteed for minimum 5 years.
Are there some comparison between AppSupport and Waydroid use on the same phone, performance-wise?
The only threads I found with some info are WayDroid on SFOS? (mostly about installation and making it work), Waydroid vs. AlienDalvik & AD history (some technical differences and a few issues but still nothing on performances), and Performance Under Par (talks about performances but Alien Dalvik and AppSupport only mentioned together to remind the difference).
Regarding root-detection, is there a notable difference between AppSupport and Waydroid?
Once again, I did not found any direct comparison.
Somewhat hidden through [Sailfish Community News, 21st October 2021, 10 Year Announcements : Sailfish OS 4.3.0 will have a signed system image for Android App Support. This should allow apps that previously complained about not running on a rooted device to work correctly.
It is not a comparison, but does address the question.
So regarding direct comparison of AAS and Waydroid regarding performances, if you have any link, I will happily check there. Also, it is entirely possible that no-one performed these comparison, which is fine, I’m just wondering if I searched with the wrong keywords (I did not try with “Alien Dalvik” at first, for instance) or if there is indeed no data.