Thinking a bit about this, the "both" option can work but it depends on how the group setting is structured. If you segregate the member notifications to manual vs automatic and arrange the two options as such, it will work, i.e. that group notify is a child to the Auto-Lock (see below). If it is a top-level option it would be impractical, as if you wanted to lock a topic you'd have to go to MessagePolicies first to turn notification ON (if you had it to OFF so as not to notify when topics are auto-locked), lock the topic).
Which means, when locking a topic manually, a confirmation dialog would be required but it would be many clicks less than the above case.
IMO, the attached can work quite flexibly for having both per group (auto) and per topic (manual) locking member notifications:
A: If you want to notify the members when topics are automatically locked by the system due to duration (message- or hashtag-induced) set the group auto-notify ON, otherwise nothing changes/no impact on anyone, because the default is OFF.
B: If you want to notify the members when you manually locks a topic, use the Topic-notify checkbox when locking that topic, which is independent of the group setting.
This gives you the best of both worlds. But if I had to chose between the two, I'd personally also take the manual/per topic locking with the confirmation.