Would it be considered corruption if an incumbent US Congressperson used their personal finances to donate to a political organization?

It depends on the nature of the donation. If there is a clear quid-pro-quo arrangement going on, such as the congressperson using the funding to support a lobbying group in order to court them into hiring him when his term is up, this can be seen as corruption. However, for the most part, corruption is in the other direction, e.g. a political organization gives money or a non-monetary good to a politician for some political benefit.