I hadn’t heard of thredup but looking at it think it’s just in the US.

I’m similar age to you I think and can confirm middle aged spread is definitely a thing, I used to be super skinny but not recently and my pants definitely fit more snuggly. Personally I’m just gradually replacing stuff a bit more roomier fit as I go. No big revamps for me. I’m also rediscovering brighter colours for some reason and getting compliments on that. I think I may be a more casual dresser than you describe however.