We struggled with this for years. It was fine while we both used the same Apple ID, but when my wife got a watch we had to move to separate Apple IDs (health data got merged), and our single shared photo album no longer worked.
I tried various things but the answer which works brilliantly for us, but costs, is Lightroom, the cloudy version (not Classic). It is installed on all our Devices and two Macs, and works exactly like Photos did when we used the same Apple ID, but now we are both signed in to the same Adobe ID.
Pictures we both take on our phones, or added from a camera, are automatically uploaded to the Adobe Cloud and sync’d to all devices. Edits and culls on any device are automatically shared across all devices.
We have 79,000 pics in our Lightroom library and available on all our devices. Unlike Apple, Adobe uses smart previews so that you don’t have to have the whole 600GB on any device (but you can keep a local copy if you want). The originals are all in the cloud.
PS There is a real demand for this, but Apple don’t provide a solution, so I have to give my money (£10 per month) to Adobe instead of Apple.
PPS Many people don’t realise that Lightroom (Cloudy) is a completely different and independent app from Lightroom Classic. Adobe have confused the hell out of everyone by giving them same name and allowing some complex syncing between Classic and Cloudy which is not needed.