Containers are awesome. Still waiting, though, for when I can set bookmarks to use a specific container. I have a few different gmail accounts and it would make accessing them a lot easier (having containers at all is already a huge improvement).
I use pinned tabs for a similar use case (o365). Open the page in the container you want, right -click -> pin tab and it will remain on the left of the tab bar with just the icon remaining.
Yup, that's exactly what I do right now as well.
Click ••• > Always open this in a Container > Pick one. Done.
As I mentioned, it's for multiple different gmail accounts. Same url, different containers.
you might be able to trick around that if you can set up something that redirects. I have multiple bookmarks that go via the same URL via a redirect (it's an SSO login flow), but if I set the initial URL to be tied to a specific container, the entire flow remains in that container. So if you could set gmail.example.org to to redirect to gmail and gmail2.example.org as well, you could tie those urls to a container and use bookmarks. It's a bit roundabout, but it probably works.