Mac Photos - How to make Permanent on Mac

Now to the backup part:

• On my Macs I am keeping an archive with all my photos and videos on an external drive and a cloned copy of this archive. This archive is split into several smaller Photos Libraries (plus the original Aperture and iPhoto Libraries). One library for each year, and a separate library for each major photo safari. Keeping the archive structured into smaller libraries makes it easier to make a backup and faster to work with it. To move the new photos to the archive I am using PowerPhotos. It can merge selected albums from one library into another library.
• Optional, but very useful:  I am keeping an additional archive of all original photos, where I am saving the new photos, right before importing them to Photos, directly from Houdah Geo after tagging them with GPS and basic keywords. This archive is just a set of nested folders, Year > Month > Day.This way I can recover missing or corrupted original without having to open an archive library.

Which procedure will work best for you will depend on your priorities and the local storage on your devices. As long as you do not need iCloud for syncing and hand-off or want it as an off-site storage, don't use it, because it will just make it harder to  backup the photos library. I have loved iCloud Photos as a syncing service right from its first release in 2015. But all new photos are first archived outside of iCloud and backed up. See: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community

There is no good way to make backups of an iCloud Photos Library when using "Optimize Storage" for iCloud: How to back up an optimized iCloud Photos Library - Apple Community

Even with "Optimize Mac Storage" disabled, it is difficult to check, if all originals have been downloaded from iCloud - the status message "synced just now" does not ensure, that all originals have been downloaded and the library is complete. Here Power Photos is coming in handy. It will give a list of photos with missing downloads, when it copies a library and we can force the download from iCloud for the missing items and repeat the transfer of the new phots to the archive libraries.

I use cable sync and macOS Photos app and no iCloud Photos, with backups to parallel Time Machine, and occasionally to other media.

As soon as iCloud Photos is enabled, that then starts managing storage and caching. Given your goals, you’ll be tussling with that for the foreseeable future then, too.

Or probably better still, find a photo storage or photo library app that better meets your needs, and dedicated archiving processes.

It will depend on your devices and the way you are working with them, how to organize your Photos Libraries and to solve the different tasks of importing and syncing your photos, archiving them and backing them up - my main reason for sticking with Photos for Mac is the benefit offing able to use iCloud Photos for syncing and transferring photos between my many Apple devices.

As long as you are not needing all the goodies of iCloud Photos, you are fine with your plan to get your photos archived on external drives. I have become spoiled by the ease of use of iCloud Photos and could no longer do with it. My approach:

• I am using iCloud Photos solely as a syncing service for my current working set of photos. This way I can benefit from the hand-off and syncing. All new photos and videos are appearing instantly on my five Macs (with different system versions from Mojave to Sonoma), my iPhone, my iPad, my Apple Vision Pro, and my library is available on my Apple TV. I can start working with my photos on any device that I am just having near me and continue working on any other device, when I go to a different room. All albums, adjustments, metadata are kept identical across all devices. iCloud Photos is the thing I am enjoying most in Photos, particularly now, that we can split our iCloud Photos Library into two sections, a private library (unfinished projects and new photos) and a shared library (the show case). (Using the Shared iCloud Photos Library to have two separate libraries on all devices - Apple Community) Previously, with Aperture and iPhoto, when we had only My Photo Stream to import automatically, it has been like herding cats to keep the photo libraries identical on all devices, as we could neither sync the albums nor the edited versions with the adjustments and metadata, and Photo Stream kept the photos just for thirty days and only thousand items at a time. When I have been traveling for several weeks, I had a lot of work to get all new photos to all devices and to avoid duplicates, as Photo Stream had only transferred a random subset of my photos. I have been waiting for a decent syncing service like iCloud Photos for many years.
• My iCloud Photos Library is holding only my favorite photos and videos and the new items I am currently working or using for some project. It is small enough to fit on all devices, without having to fall back on "Optimize storage". The Macs I am using to work with Photos and my iPad are having 2TB of storage, the iPhone only 1TB, but the library is small enough, so that will suffice as well. It helps to keep the iCloud Photos Library small and focussed on the favorites and the current working set, because this way it is much easier to use on the mobile devices. On the mobile devices it is tedious to have to scroll through long lists of albums. The iCloud Photos Library is included into the Time Machine backup on three different Macs. The Time Machine backup is working, as long as I do not use "Optimize Mac Storage". And I like to have my favorite items in iCloud as well, in an off-site storage.

I have to split this post - more than 5000 characters

You're welcome, Paul.

Your NAS will be very useful to archive photos and videos in folders, but as you probably know, you cannot store the Photos Library on a NAS, because a Photos Library needs to be on a locally mounted volume. The internal database files in the Photos Library might get damaged by transaction errors, if you access the photos library over the local network. Move your Photos library to save space on your Mac - Apple Support

You can't store your library on a storage device used for Time Machine backups. And to avoid possible data loss, don't store your library on a removable storage device like an SD card or USB flash drive, or on a device shared over your network or the internet, including over a cloud-based storage service.

I would back your photos up to a external SSD. I wouldn't bother with iCloud unless you want them there.