Question regarding general Spring Boot application architecture

There are two external APIs that I have to consume in my app, parse the data and serve it to front-end. The APIs have different response and request objects and one handles XML and the other one handles JSON and front-end has the possibility to see the data coming from both external APIs.

My question is what would be the best approach to do build this app? How should I structure the code, bearing in mind that another APIs could be added in the future, so I would like to make it modular as possible, but I'm not sure how to achieve this.

Should I go for microservices architecture, where I have the main app and it communicates through Feign, where API1 is one service with the models and services etc and API2 is the other microservice and front end is hitting those microservices separately? Or should I have one monolith where I have both API1 and API2 service layers, controllers, models etc and front end is hitting the monolith where the business logic determines which API service/model/controller to use?

So something like this..
**
API1 Microservice/**
API1 model
API1 service
..
**
API2 Microservice/**
API2 model
API2 service
..

**Monolith**:

Services/ (Feign implementations, other business logic etc)
    API1 service 
    API2 service

Model/
    API1request
    API1response
    API2request
    API2response

Controller/
   API1controller
   API2 controller

I'm able to actually build the app and write the code but would love some input as to how I should organise the actual app and possible make it as modular as possible.

Thanks.