Thank @Zesko
I will try and do it.
As far as I know BTRFS would me much faster in taking Timeshift snapshots (faster boot time perhaps, faster system?.. I read a lot of pros and cons about BTRFS, I better here from an expert)
It also is possible to convert a partition from ext4 to btrfs 
But be aware that you have to follow the book !
(no playing around while converting !)
Don’t worry about btrfs. ext4 is just fine. And if anything it’s also faster.
Thanks @zbe
I read again about BTRFS, my final conclusion: it may have good new features -but at a price- and EXT4 is not “bad”.