Is this a fine graphics card?

@mollythemaid  Reshade will increase the demands the game places on your graphics chip, but I think you should still be able to get good performance.  If you're noticing an impact while using Reshade, turn down the in-game settings a bit, or try a less demanding preset; from what I understand, there are plenty of options.  There's no harm in experimenting a bit; I'm sure you'll be able to find settings that work for you.

Custom content in general should also be fine, but there are plenty of items that are much more demanding than EA-made content.  Here again, if you're noticing a performance hit, you may want to look for cc items with a lower poly count.  Most responsible cc creators will list the poly count on their download pages: the more polygons an item has, the more resources it takes to render.  Here again, there's no harm in downloading something to see whether it works to your satisfaction.

Storing your custom content on an external drive will actually hurt loading times and won't help performance in any way.  Your processor and graphics chip will be doing the same work to render the items, but the computer will be calling the data from what is probably a significantly slower drive than the internal one.  So if your computer has the space internally, I'd store all game-related files there and use the external for other data, for example backup saves or collections of photos or music.