Is it possible to change the system font to a monospaced font? Due to declining eyesight, monospaced fonts are way easier for me to read. Is there a way to switch the default font High Sierra uses out for one that's monospaced? If it's not supported natively, are there any 3rd party programs that can?
Unfortunately there is no built-in way to change the system default font, there are, however, a couple of utilities that let you change the default system font or at least some of the system fonts. How well this will work with a monospaced font like Courier is anyone's guess, and mine would be probably not very well.

You can change the font back to something many thought was more readable than the current font. By changing it to Lucida Grande.

There is another utility called TinkerTool that lets you change the font for some of the macOS. I don't believe it will change everything but it likely worth the try.

Those are all I know of, there may be others that work better but these should give you a good start.