For AutoMapper: consider Mapster or manual mapping (my recommendation).
For MediatR: explore FastEndpoints or build a simple mediator yourself.
For MassTransit: look at raw client libraries like RabbitMQ.Client and Azure.Messaging.ServiceBus, and another option to consider is Rebus.
Write equivalent functionality yourself
MediatR isn’t too complex to build on your own. I recommend giving it a try as an excellent coding exercise - it’s probably the simplest way to move away from MediatR.
For AutoMapper, many teams have deep integrations with business logic in custom mappers. This makes extracting and replacing it difficult. Expect significant tech debt if you don’t address this.
MassTransit, on the other hand, does so many things (and does them well) that migrating away would be challenging. Saga support or the request-response messaging features are hard to replicate. The only real alternative is diving into raw client libraries for your chosen message transport.
