Defining a Message Passing domain with very many message types

Most F# Message Passing examples I've seen so far are working with 2-4 message types, and are able to utilize pattern matching to direct each message to its proper handler function.

For my application, I need hundreds of unique message types due to the different nature of their handling and required parameters. So far, each message type is its own record type with a marker interface attached, because including hundreds of types in a single discriminated union would not be very pretty - and neither would the pattern matching of these be. As a result, I'm currently using reflection to find the correct handler functions of messages.

Is there a better, and more functional way of doing this? Perhaps even a smarter way to define such a domain? I'd like to enforce as much correctness as possible at compile time, but currently I'm finding the handler functions based on a custom attribute, as well as checking their signature at run time. As far as I know, I cannot enforce a function's signature with a .NET custom attribute, and since there are too many types to realistically pattern match, I can't (to my knowledge) use a single generic message handler function either.

I have thought about the possibility of attaching the handler functions to their respective record type as a member, which would circumvent the need for reflection and enforce some additional correctness at compile time. However, it doesn't make much sense to have all those functions present client side.