Common property is just that, usage for all residents. As I understand the SMMA, nothing can be stored on any part of common property. The Executive Committee’s role is to maintain and repair all common property, they do not have the authority to ask you to sign any agreement. However, at a general meeting the majority of owners can agree to allow you to use a section of common property. Generally, this may not a good solution as it can promote abuse and misuse of the common property.
I would check if those residents above you that are using common property for pot plants etc. do have the right to do so, e.g. do they have an exclusive use of that part of the property as part of their title? OH&S is a big issue for strata these days, and your point about pots falling, is valid. Maybe there is room for negotiation ?
Finally, in order to move forward, is there any area that can be set aside for safe, secure storage of bikes (of all types) with racks ? If so, before the next AGM get quotes for racks and put in on the agenda.