Not sure what might have happened
There really is no reason to “double-click” and reinstall every single add on.
- On the old computer, I’d make sure all the addons are up to date.
- Since you are able to find the old addons folder I’d copy that to the USB drive. Copy everything.
- One the new computer locates the addons folder and copy everything from the old computer’s addons folder to the new addons folder.
- when RapidWeaver starts again you should have all the addons.
Since you are running Catalina, some of the [older plugins may not work you should get a warning message about the plugins not being notarized… All the ones you have mentioned (Pluskit, sitemap, and stacks) all have versions that will work with Catalina.