Instead of Docker , no doubt its great but it has vulnerabilitis and restricitions with dameon and root thread. I would pickup Podman. Also Ambasador is a culmination of Gateway LB and ServiceMesh on istio and Envoy. Great for both East-west and North south microservices communication, policy managment and security with Istio. Spring Boot is not a WebFW. For platform web fw one can use Reactive like SPring WebFlow rather than Spring MVC. For java experience, Spring provides great assets.
I will switch to using Kubernetes whether managed or custom depends on several factors rather than AWS ecs. For LB Amabassador is a great alternative on AWS. One can simply use this on top of ECS clusters. Instead of running in to different frameworks one can simply use one FW at both client and server side for consuming and SSE. I believe one can look at Lot of it depends what you need a full FW or a light librarry like React to be part of V in your MVC. Whether you need a SPA , on Mobile etc... in that case KOTLIN is also another option on Java. Dont go with Android. Best luck. Swapnil S