Membersship start date

I need all my memberships to always renew yearly on Jan 1st
so new members added at any time during the year would all have to renew at the same time.

Take a look at this.

Also there will be no regular signup, all new members will be added manually and will only get an option to renew after.

How could I do that?

You could have them sign up using Open Registration, which would make them a Subscriber/Level 0, but wouldn’t get them access to level 1 content or above. Then you could upgrade them manually to Level 1, or whatever.