How can I optimize a very large shapefile (100+ MB) in order to convert it into a reasonably-sized KML (small enough to easily open/view in Google Earth)?

To reduce the size of a file regardless of its format, you can do a few things:

• Reduce the number of attributes, e.g. using the AttributeKeeper
• Simplify your geometries, e.g. using the Generalizer
• Convert your file into a (dumb) image.

Any of these will reduce the quality of your data. If you don't want that, you can split the file into a collection of files/tiles. See https://knowledge.safe.com/articles/887/optimizing-large-datasets-for-kml-and-google-earth.html (Beware: this is quite advanced that took me a while to master)

In the KMLStyler, there is an option Allow Unique Styles Per Feature, where you specify whether you want to have one single style for all features or a style for every feature, i.e. 10 lines of code in the header of your KML file, or 10 lines of code for every feature!

Although having one single style is often insufficient, you could create a separate KML for each style class, e.g. one for each interval or each subtype, unticking that aforementioned option!