I want my workspaces to communicate, but I don't know if it is better to put them in one space to make sure they can communicate, or if is better to create several different themes (accounts, life project, books to read...)
For me one Notion workspace with as many pages and databases you need is the best solution to have everything under the same roof.
Having different Notion accounts or workspaces can make sense if you need to actually have completely separate information that you will never share with another workspace, but if everything is for you and the main idea is to share data and information between databases and pages, just one Notion workspace is enough.
Having a different workspace for stuff creates all kinds of extra complexities, especially if you work with others on things. Sharing stuff between workspaces sometimes works a bit wonky and comments on more than one workspace will have you switching between workspaces until you lose your mind. So basically, you'll set yourself up for a lot of extra clicks and lack of overview.
So I'd say: keep your number of workspaces as low as possible UNLESS...
In all other cases, you and you sanity will thank me if you just use separate pages and subpages all from one workspace.