upgrade my road bike, compatible parts to upgrade in phases?
I received a used road bike that functions, but not well.


Just to point out: If shifting works bad it’s usually a problem with friction in the cables, bad adjustment, bent hanger or worn chain/cassette. Even a relatively cheap Tiagra group works flawlessly when properly installed and adjusted.

I don’t see many options for a partial upgrade because the cassette-spacing has gotten narrower from 9-speed to 10-speed to 11-speed. So you can’t use 11 or 10-speed shifters with 9-speed cassettes or vice-versa.

Anyways, you’d probably have to install new cables and re-adjust for every step so it doesn’t make much sense.