"How has chemotherapy evolved in the last 20 years? Is it more effective and does it have fewer side effects/discomforts than before?"
Short answer: chemotherapy isn't better, but immunotherapy shows terrific promise.
Metastatic HER2 positive breast cancer used to have a life expectancy of 2 years. Now there is no limit. I have patients with stage 4 HER2 positive breast cancer surviving going on 15 plus years.
See also "Has chemo ever actually cured a person to the extent that the cancer never returns? It seems like it just buys time."