100 amp sub-panel replacing old fuse box
I agree with Harper and RME. You do not need a sub-panel.

A junction box will work just fine especially in an attic even if it is a walk-in attic.

The National Electrical Code requires, with few exceptions, that a conductor be protected at the point where it receives its supply.

The fact that you can shut all of this off with three breakers in the basement means that they are already protected.

If you wish to add AFCI protection then just replace the breaker or breakers in question with an AFCI breaker. Much easier and cheaper.

Good luck and stay safe!