I think most teams, probably 90% of them, should start and stick with Rails conventions. Intelligently apply design principles, watch out for coupling that’s not worthwhile, carefully add dependencies when you must, sure. But don’t worry too much about erecting a wall between your app and Rails, building microservices, or whatever fashion dictates when you run
That said, I don’t think strict adherence to Rails’ opinions is the only way to succeed when using Ruby to build for the web. You can adopt the principles of Rails’ opinions, e.g. use code over configuration to fight boilerplate or reduce the number of choices developers need to make by curating some libraries. You could document those principles and invest in new teammates by mentoring them up on your framework and tools.
Actually, you should do that anyway! But there are reasons you may not be able to do that: the team is too junior, time is tight, you need to explore new technical ground in other areas of the project. If that sounds like your team, you will benefit a lot from letting Rails do much of the tool-building, principle-seeking, and training for you.