Braising the roast in the oven is our preferred method for cooking sauerbraten. I don't know if it is the "best" way, but it the way that works best for us. We’ve made sauerbraten in the slow-cooker on numerous occasions, and while it comes out tasty and delicious, it is always uniformly too well done. Mr B and I prefer our roasts - sauerbraten or otherwise - to be medium to medium rare, and nuances in temperature are much easier to achieve by braising it in the oven. I am positive that there are folks out there who prefer to make sauerbraten in a slow cooker, or their Instant Pot (or even on the stove top if you are an old-school Deutscher Koch*). If this is you, go for it!  *translation: German cook