ah, I will check out memory hole on some of my systems.
At least on 6.1 you have to be below 50% RAM usage too.
I found this in a RHEL doc that pointed to a kernel README that looked old af but said the same thing.
I have a few systems that run an app on solar during the day at 80% RAM and I had to stop the service before suspend to get it to work.
Yet it worked for a couple months in disk hibernate but then stopped and only memory sleep would work. On a Debian Bookworm stable kernel, so who the heck knows what broke (wasn't me!).
Battery usage overnight is different enough with many machines that I wish hibernate to disk worked reliably.
Anyway if I have 16GB RAM and a 36GB swap it seems bonkers to me that it is by design only working if less than 8GB of RAM is committed.
The subsystem is quite brittle and everybody seems to know.