I’m saying this based on real world experience: after a certain point you start to see deminishing returns when optimizing a system, and you’re better off focusing your efforts elsewhere. For most applications, customizing containerized services to share databases is far past that point.
Do you have the data to back that up? Have you measured how much of an impact on system load and power consumption having 2 separate DB processes has?
Roughly the same amount of work is being done by the CPU if you split your DBs between 2 servers or just use one. There might be a slight increase in memory usage, but that would only matter in a few niche applications and wouldn’t affect environmental impact.
For most applications the overhead of running a second DB server is negligible.
I write software for a living, and have worked with all 3 database options in the past. I don’t know what DB backend my nextcloud server is using, nor do I care.
You seem to be obsessed with optimising one resource at the expense of others. Time is a limited resource, and even if it only takes 5 minutes to configure all of your containers to share a single db backend (it will take longer than that even if you just have 2), you’re only going to save a few MB of RAM. And since RAM costs roughly $2.5/GB (0.25 cents/MB) your time would have to be worth very little for this to be worthwhile.
On the other hand, if you’re doing it to learn more about computers then it might be worthwhile. This is a community of hobbiests, after all…