As per my understanding, traveling from Rishikesh to Sonprayag post 12.30 pm might be challenging, especially by public transport like buses. In the hilly regions of Uttarakhand, public transportation schedules can vary, and it's common for buses and shared taxis to have fixed departure times, usually in the morning. To increase your chances of finding transportation options, it's advisable to start your journey early in the day. Shared taxis may be available but are subject to demand and availability, so there's no guarantee you'll find one later in the afternoon. Better if you could check with local agents or local people or from bus depot.