Is being able to comfortably answer the question to yourself “why do I need another bike?” a reasonable first step? I’m thinking you either need to be totally convinced yourself or invite her to participate in your decision. You start the conversation with her with something like “I’m having this debate with myself on getting N+1 and here’s what I’m debating. What can you add?”

In my case, My latest additions have simply let me do things I wasn’t able to do with any of the current fleet which meant adding new adventures and activities.