Design options: multi clusters, multi instances or multi availability groups

We have a Microsoft platform, where within it, we have two sets of databases. One set of databases for the platform services etc. and one set of databases of more custom nature with some data that is loosely connected to other databases. We would like to keep these database sets separate, as we would prefer that the platform set of databases has more performance than the other custom databases, but it is not the most important factor – high availability, stability, best practices and little complexity/ease of use is more important. When we brainstorm this, we come up with three alternatives: - Option A: 2 clusters, 2 availability groups This is our current setup, obviously the best (?) but also most expensive and most time consuming to maintain - Option B: 1 big cluster, 1 default instance, 2 availability groups Should be the easiest to maintain, but what options do we have to ensure our “preferred” AG (with platform databases) has highest availability? - Option C: 1 big cluster, 2 instances, 2 availability groups With this option we can customize all the hardware resources per instance we want, but multi-instance, multi-AG also seems a bit complex to maintain We would like to hear from others if they have opinions on the options above? What would you choose if money is no option? Any other options available?