It has been quite some time since I last reported my experience in this thread.
Good news: since I applied “the two noticeable changes in my habits” (cf my previous message), my Framework 16 laptop successfully wakes up from s2idle sleep every time.
Of course, it is also possible that something was fixed by a kernel update (the laptop runs Debian Sid).
Bad news: I experienced the lid-hitting-the-keyboard-or-touchpad issue multiple times, both at home when the cat walks on the laptop (cat-proofing is the next step after dog-fooding) and when traveling. Regarding the latter, system logs confirm the laptop typically wakes up when I start walking. Depending on how long I walk and what is running on my laptop, it may or may not be burning hot when I finally take it out of my bagpack.
I intend to investigate whether I can configure the exact kind of events that wake up my system so as to work around that issue, but a firmware update that ignores all Framework input modules when the lid is closed would make sense.