You can upgrade instantly on AWS, it doesn't take weeks. A lot of problems with AWS Elasticsearch Service are caused by corporations trying to squeeze a dollar out of a dime.

T2 and T3s with no Master Nodes, let alone three, leads to a lot of problems.

When everything lives in MMAP'd memory and you are nickel and dime'n it with 4GB and one CPU. You're going to have a bad day.