If you really want big external drive capable of being used safely(!) at least by both Linux and macOS :
What is the best and safest format to use for
big files/backups/disk images etc?
Is it possible at all? I heard many bad things about ExFat, that is not safe and invitation for failures? But what is the alternative then? Do we have one in 2021?
Is it practical to use zfs for instance?
In general, what is the safest format to use for storage
that will be readable in some observable future by different OSes?
This is very generic. The main issue is that it doesn't support journaling so crashes / power loss could affect it. I'd use it for a shared internal drive, but if your point is to have it as an external/portable one, it could lead to consistency issues.
If you're happy to spend some money, there's always extfs support https://www.paragon-software.com/home/extfs-mac/# or NTFS which is well supported on Linux and has a commercial driver from Paragon: https://www.paragon-software.com/home/ntfs-mac/