We moved from Heroku to GCP after approximately two years of using Heroku. (This was three years ago so some information may have changed)
The move worked out incredibly smoothly and has saved us money and allowed us to "modernise" our infrastructure to take advantage of some of the newer trends in Infrastructure and Security.
To address your direct questions:
1. Not very long. We were running a NodeJS app with a web layer and several background workers. We were able to get this running on a Google Compute Engine VM in about 1 day using Packer. The whole migration process took about two weeks start to finish.
2. Our team is relatively experienced and had experience with all three major platforms and Kubernetes (although we chose not to use Kube in this case). We are definitely a team of developers, not sysadmins though. This means we had to learn some new things particularly about tuning NodeJS apps on raw linux.
3. I don't think we learnt too much (other than the undocumented rough edges of both platforms) but it was definitely worth it for financial and quality reasons.
4. It's a relatively hard metric to calculate when the company is growing user base and features quickly - but I would estimate it at around 50%.
5. 1 app with around 5000 requests per second. NodeJS / Typescript / Rust
6. If you have only ever used Heroku I think it would be worth getting comfortable with Containers (Docker basically) and making your app run in a container. From there you have tools like Railway (https://railway.app) or Cloud66 (https://www.cloud66.com) that can do most of the rest for you.