nathand

I would start by seeing if the 1x will work better for you with a different chainring or cassette - that's much simpler.

Does your S2 frame have the right cable guides for a front derailleur? I can't tell from the photos on the Jamis site. If so, you'll need a crankset, front derailleur, and left shifter. You might also need a new chain if you're adding larger chainrings. You don't need a new bottom bracket.

This unfortunately requires a fair amount of work because you'll have to unwrap the handlebar tape and disconnect, then reconnect, the hydraulic brake lines. Personally I'd have a shop do that, although I don't think it's difficult with the right tools. Installing the derailleur and crankset is simple enough, doesn't require much in the way of specialized tools: hex keys, screwdriver, chain tool, and the tool to remove the plastic preload bolt on the crank.

By the time you've gotten through the cost of the parts and labor for this you might be better off selling it and buying the S3 version.