If I want to write server api, I will use NestJS as primary framework. It base on express | fastify so I can use anything of expressJs. SpringBoot is a java framework. I will think to write as payment transaction service. If you are Ionic developer. you will know Js / ES6. You can try NestJS / ExpressJS.

• video calling: You can not create video calling feature. Some keywords: ffmpeg / coturn / webrtc
• chatting: realtime (socket.io / websocket) - you can try some opensource as rocketchat. It also have video calling feature.