the only way to keep things in sync together would be to use the same icloud account, but that gets messy. my parents do this, each device they own has the same apple id for icloud and for the app store. so if my dad takes a pic on his iphone it'll show up in his camera roll and also my mom's bc it's the same id. They also use the same contacts and calendar so it's not a problem for them. But this is where apple fails IMO, there is no family shared icloud account for photos. We have a 2TB icloud plan where we keep all of our stuff backed up, but we also pay for dropbox so we will both have access to all of our photos. I'm the one that takes a majority of the photos when we go on trips so they are always in my camera roll. Many years ago i had created a dropbox account due to the lack of sharing on apple photos. Everything is organized into folders by type, aka vacation - then by state etc. So with that, my wife has dropbox and can access the photos quickly and we can both "share" all photos. It's not as easy if you're on apple photos. I've even gone as far as created picture folders in "files" folder in icloud drive, but it's extremely slow when sharing and is just overall pretty terrible to look at pics that way. So for us, apple photos isn't a completely solution.
We have optimize storage turned on on our phones and laptop, but we do have the 2TB plan so everything backs up to there. but again, there's not an easy way for my wife to see the photos on my camera roll so that's why we have dropbox as an external solution and to actually act as a 2nd backup so to speak.
The downsize about Apple photos and iCloud, is that when you have enough storage space, you either have to optimize the storage on your phone or have a phone with a big enough hard drive to handle the storage of the pics. So it doesn't work like dropbox where you can manually or even automatically upload them to dropbox and then you could delete them from your device if you wanted to. If you want your pics in apple photos, they remain inside your camera roll...so no deleting them.
What you could do is do a family plan iCloud plan and share 2TB between you, but you'll have to optimize the storage on your phones and there won't be anyway to easily share your photo library with hers unless you start creating shared albums, which is easy going forward, but if you have 400gb, that would take absolutely forever.
