I don’t know of a political movement yet, but I would guess the following:

• Without Russia, the new St. Petersburg country could engage with more trade with the EU instead. The region is one of the more advanced parts of Russia, so they may figure that they could do better trading within the EU customs union than being frozen out.
• While a lot of resource wealth goes into Moscow and St. Petersburg from some parts of Russia, there are a lot of other areas that rely on massive Russian subsidies to maintain their standards of living. If resource wealth gets cut off, St. Petersburg may not want to subsidize these backwater areas.
• The Wagner Group is located in St. Petersburg. It is possible that a future civil war between the Wagner Group and the Russian government may lead to the Wagner Group wanting independence from Russia to keep its pseudo colonial riches.