Got my AWS architect cert 2 weeks ago.

What you can do is setup a spot fleet so it'll fill up with spots and only use on-demand if spot goes above the on demand price.

You could also have a pure spot fleet and a reserved instance and use a load balancer with health checks to route traffic.

The one thing you shouldn't do with cloud providers is lift and shift your existing instances, that's what leads to the crazy prices some people are seeing.

Renting an ec2 on demande and installing your software is almost always the wrong way to do it.