Yes if you have a big HDD for speed try to get bcachefs as Filesystem. It is somewhat complex but if you install a small 250GB or so SSD you could speed up your read/write time a lot.
Not sure what you mean with those partitions, do you have a 6TB windows partition?? Then just delete that. And 4TB of also NTFS files?
I dont know if you can convert filesystems while leaving the files there, interesting question. But probably not