I think that Plan B assumes that i have a linux pc and a second hard drive with at least 2.5 tb (the data that i need to backup). I don't have the 2 conditions.
Then plan a : I will follow some tutorials on using parted that i found on the net. If it works thats great. If no, i will only loose my library. I will survive
I see a lot of peaple here having the same issue with ntfs. If my experience works, i will share the steps. Maybe it wil help some of them to migrate to ext4.
Take a look to the Ventoy project, you can make your thumb drive to boot almost any (bootable) ISO file.
What you want to do, with resizing, moving data and resizing again on same disk, is risky. If anything go wrong you will lose all your 2.5TB data. So if you have a possibility, save your data somewhere else, boot a live linux (GParted, Ubuntu...) and change the partition to EXT4, than copy back your data.
If you will try the "plan a", the risky way, for NTFS partition resize, the MiniTool Partition Manager installed on Windows PC is a good idea, but for creating/copying/resizing the EXT4 I recommend to use a (live) Linux OS.
And if you that 6TB drive will want to use with Windows too, maybe you can consider to use EXFAT partition instead of EXT4. In this case you can use your windows pc and the MiniTool Partition Manager.