Hi Bill,
Since my 2017 i-Mac is Very, very slow with too much old S/W on it (ETRE Report attached) I think I need to do a clean install. I presume the options to do that are clearly presented when I install the latest Monterey S/W on the SSD. Then I would migrate manually only the apps and, data I now need.
If you're looking at a clean-install, follow the steps in the SSD User Tip for "Erasing the external SSD and Installing macOS", but stop at "Moving your content to the external SSD". From there, continue to setup your Mac as new, without migrating any information from a backup, as doing at this step will restore all the old apps and software issues.
Next, after your Mac is setup, Restore items backed up with Time Machine on Mac - Apple Support to the right locations. Additionally, you can then re-install the important apps you use from their original sources (such as the developer's website).
Should I upgrade to the latest Monterey OS now before upgrading to the external bootable SSD? I'm not sure although I've heard that some SSD's have problems loading Monterey. I have plenty of storage (500gb) so I'm not too concerned about running out of space. Going to the latest Monterey before would mean my HDD disk would be updated before going to the SSD which would be good for backup ops.
Yes, I would. Go to System Preferences > Software Update to perform the update.
Using the macOS Monterey installer later to install macOS to the external SSD will install the latest version to it.
Before your post I was seriously looking at the T7 Samsung 1 TB SSD but see some have had problems loading Monterey with it. It has a price advantage vs LaCie ($109 vs $150) but quality is important to me. I understand you have an excellent experience using LaCie SSD's.
Yes, Samsung SSDs do have attractive prices. However, like you suggested, there's been a history of issues with Samsung external SSDs being used as startup disks for Mac computers. LaCie does cost more, but the quality seems to be there, plus Apple sells it.
Another manufacturer I've had good luck with is Other World Computing (OWC). I've purchased Memory/RAM from them before. They have a new Envoy Pro Elektron SSD that may be another to consider. I haven't tried the Elektron SSD personally, however I am in need of a new external SSD, and I'm tempted to try the new OWC solution.
I currently have a LaCie Rugged 2TB SSD and I don't have any issues.
Of course I have Time Machine backup's. Will they be compatible if I go to the latest Monterey?
Yes.
If you have any more questions, please ask!
Cheers,
Jack