When I last rode the train semi-regularly I felt like I had to complete a PhD in train ticketing. I put at least 3 hours into researching the cheapest ticket and eventually, after riding for some time, discovered an even cheaper option.

In the end the cheapest option was not to use a BahnCard 25 or 50 but to buy two one-way tickets for each leg from the regional transportation provider via a smartphone app, because smartphone tickets are sold at a discount. It's wasn't just a bit cheaper but about 30%.