Install macOS High Sierra on 2011 iMac SSD

FYI, if you use Internet Recovery Mode, then you may need to use Fix #3 in the following article if you get a "the recovery server could not be found" error:

https://mrmacintosh.com/how-to-fix-the-recovery-server-could-not-be-contacted-error-high-sierra-recovery-is-still-online-but-broken/

If your old drive still works, then you can create a bootable macOS USB High Sierra USB installer using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

When erasing the new drive, make sure to erase the whole physical drive as GUID partition and MacOS Extended (Journaled) so that a new partition table is also created. Within Disk Utility click "View" and select "Show All Devices" so the physical drives appear on the left pane of Disk Utility.

Hi,

Start up from Recovery Mode could install manOS on your SSD.

How to start up from macOS Recovery - Apple Support

That modification will significantly increase the performance of that Mac, but High Sierra is barely useful for anything Internet-related these days. A much better alternative would be to repurpose that Mac as a Linux workstation, which will remain viable for years to come.

Linux Mint makes it easy:

https://www.linuxmint.com/

Having said that Linux is so resource-efficient it will work impressively well even with that Mac's original hard disk drive.

The OWC Mercury Pro 6G is an excellent SSD, superior to anything else on the market. They are the only ones I have used and can personally recommend. The oldest of mine are significantly older than your Mac and still work as well as they did when they were new.

The real question here is "why"?

That machine is obsolete. So obsolete that eat complete working machine with macOS High Sierra installed and running is worth less than the drive you want to install on it.

Don't bother.