Heroku would be the easiest to use, and it has fixed costs attached (unless you use autoscaling, which scales worker based on latency.)

AWS AppRunner (built on Fargate technology) would be another option, but using AWS is definitely more involved than Heroku. With the exception of network traffic which is pay-as-yo-go, you can calculate max pricing for CPU/memory depending on the app configuration.

I use both of them at $dayjob. Unless there are good reasons to choose AWS, I would always recommend to start with Heroku.