How to evaluate accuracy of a regression model

… the model features are currently linear

As far as I have understood your first post, you have used a linear model.

In this case it would be helpful to model the non-linearity already in your features with feature engineering techniques. I think this is also what you are saying with polynomial features, right?

Also here the residual analysis might be a helpful tool to check how successful you were also with respect to systematics in your residuum as mentioned in the previous post. If you see systematics you can design your features w/ mathematical operations (e.g. with polynomial approaches as you mentioned before).

Anyway - good luck! Feel free to post an update or ask any question if something is open or unclear.

Best regards
Christian