Hi Blythe,
the Windows version where you GP client runs matters... if you're on GP 2016, Win2019 wouldn't be a supported configuration, though I'm not saying it's not working.. So RDS 2016 it would be.
For the SQL server, your Windows 2019 should be fine and with some tricks you can get your GP 2016 databases up and running even on a SQL 2019 server.. SQL 2016 has been around for a while already and will be going out of mainstream support soon (if not already).
That being said, if you're currently running GP on prem, why not just keep that config until you decide to upgrade to a newer GP version (i.e. 18.4) ? What is the idea behind moving your on-prem GP instance to the Cloud ? saving money ? this works in the case your current infrastructure is aging and needs replacement soon with heavy investments... otherwise, just keep your infra and keep running GP on-prem. Build a solid DR plan around it and your boss will be happy.
Moving GP to the cloud isn't cheap on the long run, though it depends on how big your setup is and how many users you have.. but count a minimum of 500$ a month (and that's a small config).
I'm supporting several clients on Azure with GP in the cloud and the monthly costs ranges from 750-3000$...
HTH.
