My question: Is it worth upgrading your league (from Silver to Gold for example) on the last day of the season so that your next season chests are in the higher league? (I would continue playing in the Silver league in this example)

Regarding this question, normally the answer is yes, but it is dependable on your deck and if you spend the majority of your season in Silver 1. If you are playing in Silver 1 for say 12-13 days out of a 15 day season, you will be able to accumulate enough Gold season chests to make it worthwhile. Especially with the new Soulbound cards multiplier, there is more incentive to get Gold season chests