You're right-- when you delete pictures from your Photos Library, as a safety they can still be found in the "Recently Deleted" view for 30 days, and Recently Deleted can also be accessed from any connected device. However, if you were to scan through the posts here, you would see that an amazing number say, paradoxically, "How can I recover my permanently deleted pictures." Many people defeat the safety feature by deleting pictures from Recently Deleted, and then regret it. So-- the warning. Also, some disasters occur after 30 days.
Also, Photos is a non-destructive editor, so even if you do some crazy edit that you find embarrassing later, you can always revert to the original. As long as you have it.
Another thing: You can use "Optimize Storage" on the Mac, on your iPhone, or on your iPad. If you set a device to "Optimize Storage," then Photos may store only smaller images on the device and rely on iCloud to keep the full sized images. So, if Optimize is chosen, and you want to magnify a picture to see more detail, Photos will reach out to iCloud to get a full sized image for you to zoom in on. It's the same for editing or printing or anything that demands the full picture. Your optimized Library may take up less than 20% of the space of a fully downloaded Library.
I use "Optimize Storage" on my phone, but never on my Mac. If you "Optimize" on the Mac, then backups will only preserve the smaller images and not the originals.
I use Time Machine to back up my Mac. Time Machine does an incremental backup every hour if it's plugged in.
It copies only things that have changed so it uses very little space each time. I use a MacBook, so I only plug in the Time Machine drive at night. But if I suddenly discover that I erased a picture or file I want from last December, I can go into Time Machine and recover it. Or December 2023.
And, as you say, monthly (or so) I drag the entire Photos Library to a separate external drive as an additional backup.
Well, that's a lot! I hope it helps…