I have a B2B app. It allows Google Sign-In only. My last B2B app was the same. Nobody complained.

Understand your audience. If you can just go with something like Google, do it; you'll save a ton of time and can focus on developing features your customers are willing to actually pay money for. Authentication method is not usually a value proposition.

Only implement multi-service auth as a last resort. It's great for mass-market consumer apps, but the development workload is significant and you will end up with a support workload for merging accounts no matter how clever your implementation is.