Hi partner,
Please cover only one question in one thread, and post multiple questions in multiple threads.
- But what about how do I relate the Goals to Accounts since the Goal Owner is a lookup to user? Do I need to create the Relationship there also?
- How do I then easily in the Goal set which Account this Goal should rollup their values from?
We will use Rollup Query to specify the related Account for each Goal, provided that the entity type of Goal has an Account lookup field.

In the Rollup Query, we can set the filter criteria to include Account-related.

Documentation: https://docs.microsoft.com/en-us/dynamics365/sales-enterprise/create-edit-goal-rollup-query-sales
- What about data migration of goals, to add all the metrics and all my filtering in the rollup queries, how can a Salesmanager easily import many goal records from excel in an easy intuitive way to users?
Rollup queries are records independent of Goal, Rollup query is the lookup field of Goal entity. So we need to import rollup queries records first, and then import Goal records.
- Seems like I can only use the Out-of-the-box Goal charts with the bullet of the target and so on. Could I save as and create my own? seems not like that, the Charts doesn't get so visually good.
Yes, you can create your personal charts of Goal. (The visibility is a single user, that is, Owner.)

Or you can create system charts of Goal. (The visibility is all users in the organization.)

- How can I visibly show the Goal per months, for e.g if I set a goal from Jan to June on Opportunity Revenue, i want in the horizontal to view the "Months" and how the goal is going related to each month and the ongoing progress.
I did not find a way to display the monthly Opportunity Revenue in Goal. You can post another question on this issue and let other experts participate in the discussion.
Finally, please try not to post multiple questions in a single thread. You can post multiple threads, and each thread contains one question, which can attract more users to participate in the discussion.