You can edit an existing country, be aware your changes might get overriden in a future update though, so make a backup of whatever you’re doing.
If you look further down in the text file you found you’ll notice a list of policies implemented and their position on the slider (between 0 and 1) - screenshot below showing the policies section of usa.txt.
You can adjust, remove and add whatever you like.
Use the following file if you want to get a list of the internal policy names, this file will also show you the effects and the equations by which they are calculated, including cost / income values:
If you need help visualising the formula then a graphing calculator can help (such as this one Best Free Online Graphing Calculator - [ 2019 TI 84 Plus - 3D Download ] -)
Of course any changes to the policies.csv will carry over to whichever country you are playing.
You can adjust the value of things for your country via scripts (usa\scripts) or the effects between things via overrides (usa\overrides).
Of course changes to the game files won’t be uploadable as mods, so if you plan to upload anything, you’ll need to follow the proper process. You could take the base-game usa as a template.
I’d suggest studying the official modding guide: Democracy 4, but if you still have questions just shout.