When faced with a ball of mud or spaghetti I recommend putting all what can be factored out in a micro-service and connect them via RPC to REST endpoints. Make your services are stateless so you can scale effortless and put an API gateway in the center so you can manage. As the service business logic isn‘t really stateless put your state in a caching server. Actually you can‘t trust a cache so also put in an opaque field into your client interface. Any problems you encounter can be solved however considering the number of services to be build a fully dynamic CI/CD pipeline is a good investment for potential developer demand first.