Yah, you can't install AWS-unsupported extensions in RDS, but you can extend the type system and operators. You also have access to stored procedures with plsql/plv8/plperl so it doesn't end up being that bad in the end.
If the extension availability is the main concern, I'd recommend the open source StackGres [1] operator, which has, possibly, the largest Postgres extension catalog [2] available.
[1] https://stackgres.io [2] https://stackgres.io/extensions/
Disclaimer: founder of OnGres, the company behind StackGres