You only become invisible in middle age if a) you believe the shit about women having a sell-by date and b) you hang around with other people who believe that shit too.
It's totally a mindset thing. If you surround yourself with narrow-minded arseholes your perception will be funnelled through them.
I think this line in your OP worrying:
I'm fairly attractive and I'm worried this is my main redeeming feature and I'm boring otherwise!
Think about this statement and what it says about you:
a) You're fairly attractive: what does attractive mean? If you think it means you have nice hair, a slim figure, nice nails etc then yes you will have a sell-by date. Emphasis on those sorts of values is a fool's errand. It won't survive middle age (and it probably won't survive having children). Attractiveness is much more than that. No one ever became attractive because of how well they have presented themselves. It's your personality, your confidence, your intelligence, what you project about yourself, your interests and your values
b) Who gets to decide if you're attractive? I'm 50 and I have never felt as attractive as I am now. Because I have confidence and independence and I know what I want and I like myself. My skin may not be in the prime condition that it was when I was 22 but by God I like myself a lot more. I don't allow my value to be defined by what some random bloke thinks or how I measure up to other women in my office or what women on Instagram look like.
c) Are you really boring other than your looks? I very much doubt it. There's always something interesting about people, its just a case of teasing it out. Who says your boring? Or which inner voice are you listening to? Because you need to shut it/them out
d) If you are genuinely boring other than your looks then now is the time to start finding out who you are. Cultivate some interests. Educate yourself. Find friends who aren't shallow and who will genuinely support you.
Again, its all in the head, this stuff and it sounds as if you're your own worst enemy. Sort out the way you see yourself and the rest will fall into place.