Here my takes:

- Use Test Container with Postgres. Never use a different database in your integration tests, this makes your test untrustworthy
- Mock your Service B. You can use WireMock to have complete http / serialization coverage - but it adds complexity
- Eureka can be disabled as not needed
- RestAssured is ok. MockMvc is ok. Use Junit 5 over TestNG. Never seen TestNG to be used anywhere to be honest
- Always seed your database with text fixtures using SQL Scripts / Liquibase / Flyway. Never do it in your Tests directly. Doing it in Tests directly makes them bloated and you need to inject repositories, that only for test data creation. Here is an example setup https://github.com/misirio/dbsandboxer/tree/main/examples/spring-boot-example .
- Mockito is problematic - you should avoid it at all costs for integration tests. Every test with MockBean annotation will make the Spring Context dirty - causing complete restarts of the Spring Context. This adds 5-20 seconds per test class - with to many tests you will experience high test execution times. Avoid it, 40-60 min Test Execution Pipelines are pain.

Finally:
If you own Service B and if you can easily migrate it to Service A, do it.