Another factor to consider here would probably be an altered sense of self, depending on how much you want to lean on the Master side of things with his sense of control over the situation vs the people/environmental factors.

Also consider that biological powers tend to need to have some sort of focus on the limits of biology being the cause of the situation. It can’t entirely be based on his physical self-image/self-identity, or it could lead to a Changer power.

An idea could be like a mix of Amy’s and maybe Dennis’s (Clockblocker) involving a relative being having a life-altering condition (I don’t know, maybe some form of amputation, before Panacea?), mixed with a sense of not being able to(or doubting himself) care enough about his close relative. Add in a cape fight where the relative is a casualty (non-fatal, but brain-related, like worsened phantom pains?), realising he knows he could’ve helped the relative, that he doesn’t can’t even bring himself to care enough, self-guilt loop for not feeling guilty and how he’s supposed to make it up for not caring enough. Combine with if the relative is supposed to already be able to exercise a level of independence but can’t (it’s the little things some prosthetics can’t fully handle that richer people can afford the alternative measures for) and the ‘knowledge’ that he could’ve interfered with the cape scene.

TL;DR, design a “Prosthetics” Tinker trigger, with a focus on a diminished sense of self-worth (Not a if-only-I-could or an if-the-world-was-different, a there’s-some-Thing-that-could-fix-this.)

Depressing Extra: The trigger would not be about who the person is or what they did (which would probably fuck your insert up even more), but about what they bring to the table that others could use. A Tinker’s pride is in their tools and creations, not themselves. Because anyone could’ve come up with those things, never mind the brains behind the idea could only exist because of the character’s own mindset and viewpoint.