Hi there,
More than the frontend/mobile part, the overall architecture should be highly scalable with high availability (HA) and designed for future user growth. To this end I highly suggest going for a microservices architecture.
For reference below are microservices projects awarded/completed by me here in Freelancer:
https://www.freelancer.com/projects/nodejs/Microservices-AGENCIES/
https://www.freelancer.com/projects/dot-net/Microservices-Design/
I am a senior specialist full-stack/microservices architect/developer with over 20 years of experience, with my core languages being Java (Java EE, Spring Boot, etc.), Go/Golang, Groovy/Grails, Kotlin, .NET Core/C#, and modern ES6+ JavaScript/TypeScript, particularly Node, Nuxt/Vue, React, and Angular.
In the last two years I have been specializing with containerized microservices systems development on Docker/Kubernetes.
I am an individual/independent freelancer.
My specialized domains are banking, fintech and financial services, e-commerce, and blockchains/cryptocurrencies, so accounting/finance is right up my alley. I was also a technical lead of an accounting system for a bank.
I am also well-versed in SQL, LDAP, and NoSQL databases. My NoSQL databases experience include Apache Cassandra, Elasticsearch, MongoDB, and Redis. My cloud platforms are AWS, Microsoft Azure, GCP, and Heroku.
Please see my profile for the rest of my portfolio, work history, and qualifications.
https://www.freelancer.com/u/chesslover
Thanks.