it is really tricky to get right

Meh, rsync syntax is pretty straightforward once you get used to it. I've been running nightly server-initiated wakeonlan/full-system backup/suspend of all the machines on my LAN for years, with a grubby little cron script and rsync.
I hear backintime is nice, but I've really never needed anything beyond rsync and ssh for online/files, and clonezilla/partimage for offline/full-disk.

Fun story: I once recovered from a very dumb 'rm -rf /usr/.' (things starting with "r" and "s" are nice and far down the alphabet), without even needing to reboot the box I borked... Rsync is good, rsync is your friend, unplanned tests of backup strategy... Uhh, still count as tests?

s/filecopy/file managers that prioritise form over function and wish they were running on a smartphone/g
FTFY.

I got severely burned using rsync once, most probably my fault as it is really tricky to get right.
So I stopped using it but I rely on BackInTime which uses it without any issues.

Same here
NC was my preferred tool in those DOS/W3.11 days.

Best,

A.

anoter issue encountered:

the computer is fairly recent (1year old) > all nvme.
the NAS is fairly recent (2 year old) > HDD + 2 nvme for caching.

NAS is over 90/% full;

Devuan 6.0 (based on Trxie)
however, copy 500.000 file from Linux to NAS using SMB and file-copy via standard FileManager and the speed crawls to 1mb/s as opposed to around 100Mb/s (1GbE)(which i have) or 250 Mb/s (2.5GbE)(which NAS allows over 1 Eternet port);
still investigating; but wondered worth mentioning.

Much as I rate rsync, most of the time I actually use midnight commander

Agreed.

In my use cases:

Small files around same partition: cp or dolphin.
Remote backup: rsync
Large files or multiple files, different partitions: midnight commander

only KDE Dolphin has succeeded

Dolphin / KIO & co. have their own issues, namely being totally incapable of gracefully dealing with storage latency.

IMO, all the big GUI file managers have actually become less usable for any serious file management in recent years...
Too much focus on "safe" and "intuitive" mobile-esque interaction / notification / confirmation heavy workflows, not enough attention to fire-and-forget "just do as I say" batch operations.

Guilty admission time: Much as I rate rsync, most of the time I actually use midnight commander. Norton had file management pretty well nailed in the late '80s, everything since is just fluff.
That said;

Yeah, with 500K files I'd be using rsync. Particularly if I wanted them intact and verified.

there is also grsync to ease things up a little

actually, there is a way to "quicken up" the queue, one needs to click on the file-copy-window to "resume" the process, and the process gets worked through "concurrently".

who would have thought that;
and since the thing is greyish, must have not seen the arrows flying/standing by.

but have "apt-to-date" OS, and have now checked, with 4 GUI;

only KDE seem to copy without need of further need of user-interaction, whereas XFCE, Cinnamon, Mate, wait for another "click".

since rely on standard tools,
would say only KDE Dolphin has succeeded, but this may be open to debate.

https://ibb.co/fz6nJznB
https://ibb.co/XrGJS8PL
https://ibb.co/3mJNPySD

There are zillions of tools available for copying files, I suggest you find a better one (*cough* rsync at the CLI) or file a bug report against whatever file manager that is in that cropped screenshot.

would like to report anomaly,

since have problem with OS here upgraded to Devuan 6.0

now there are some issues during boot like

"i/o error on dev sr0, set 4, logical block phys seq 1 pvio class" or what not

that have not been able to fix so iam about to re-install Operating System;;

however, the file copy mechanism seem broken:

i am doing a backup of files; now backing up files to external HD (around 500.000 files in one folder).

at the same time, like to copy other folders onto NAS.

however, it keeps the other filecopy in queue, although the computer should be able to handle concurrent filecopy easily??

the filecopy to NAS would be over smb://

thanks.

https://ibb.co/nsTDjTNT