I'd prefer Tima as a nn for
Septima, or maybe Sep, Septa,
Sepi? Maybe
Timmy for a baby/toddler named
Septima.
Tim isn't a bad idea, but it's one of those nicknames that's incompatible with the name. So if your name is
Septima "
Tim", either your full name fits you and your nickname doesn't or your nickname fits you and your full name doesn't. I kinda feel like if your full name doesn't fit you, you should change it instead of just going by a nickname that clashes with it, because the clashing name is still there.