I think your best bet would be to drag those files to a browser loaded with Nextcloud instead of dragging them to a folder.
As you've pointed out, Nextcloud wants to sync, which means any change on either side is replicated. This is a core design goal of the software, so it's unlikely to ever get a feature which would violate that goal.
The suggestion of using scp or similar is not going to work at all out of the box and not work well even with customizations. The files on the server are stored on disk, but there's also an SQL database that indexes them. Using scp (or FTP or whatever) only gets you part way there.
If you go this route, you can put the Nextcloud command to rescan the disk in a cron job or systemd service/timer, but be warned that scanning takes quite a long time to run. So if you want to use your files on another computer within a day, this isn't going to go well. If it's just making sure you have a copy for later, then that could work.
Remore mounting is absolutely not what you want, as deleting the files would remove them from tbe server (and without removing the index at that).
Finally, if you are just looking for backups, I can recommend borg (and borgmatic if it's available for macOS). It's completely separate from Nextcloud, but it ensures you have a copy of your data. It also supports compression, encryption (client side), and de-duplication! You can also mount old snapshots of your files in case you want to just pull out one version of one file without having to restore an entire directory or machine.
Nextcloud is exatcly for this (and much more). Upload via the Nextcloud app. The mobile app can autoupload too. I use it for years
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