OF the many things, just one needs real attention:

I changed the swappiness first to 10... boom! Now it's one and the memory is half. IMO squid leaks memory!

INC4884548 i can't understand why only this machine gives this issue, and not its sister... to me this makes little sense, as it is using swap with plenty of available ram
MiB Mem :  63772.9 total,    665.7 free,  37414.9 used,  26348.4 buff/cache
MiB Swap:   8192.0 total,     36.7 free,   8155.3 used.  26358.0 avail Mem

... could it be this?

To check the swappiness value

cat /proc/sys/vm/swappiness

To change the swappiness value A temporary change (lost on reboot) with a swappiness value of 10 can be made with

sudo sysctl vm.swappiness=10