If Syntrax has been working well for you, there's really no reason to switch. Finding a protein powder you enjoy and will use consistently is more important than chasing the latest brand. If you're looking to try something different, a vanilla protein blended with frozen strawberries and half a banana makes a simple, great-tasting shake. Chocolate protein with a banana is another classic combination that's popular for a reason. You can always adjust the ingredients to suit your own taste. I also think you're making a smart move by looking beyond the ready-to-drink shakes. A tub of protein powder creates a lot less packaging waste, and it's still just as easy to make a shake when you're on the go.
You've already built a strong foundation with a wide variety of whole, nutrient-dense foods. As a nutrition counselor, I'd focus less on what you'll have to eliminate and more on following your bariatric team's post-op stages. Since you're vegan, keep prioritizing protein and add foods back gradually as your body tolerates them. I also love that you're thinking long-term instead of looking for a quick fix—that mindset, along with consistency, is what leads to lasting success.