If this is a fresh drive, reformat it taking advantage of On tuning ext4 for storage nodes, you’ll save a few bytes.
By official recommendations you should take the number your file system reports (i.e. 9.1TB), divide by 11 and multiply by 10, so that a full node will still leave 10% of disk space free. That would mean 8.2 TB. Unofficially, you can go a little further if you are careful. The amount you allocate can be changed at any time.
It is safe to remove lost+found if you have nothing important there. But even if you remove it, ext4 will recreate it at some point. Don’t bother.