Yep, if you are on Windows just extract that .exe somewhere and then run it.
The README is displayed underneath the screenshots on the link to LASIM above. It tells you how to run it. See "How it Works".
https://github.com/CMahaff/lasim#how-it-works
The latest version supports any Lemmy 0.18.3 instance. You can see which version a Lemmy instance is at the bottom of site. For example, your instance, reddthat.com, is 0.18.3. Most instances should have upgraded to 0.18.3 already - choose one of those. Any instance running 0.18.3 should work, big or small.
The account you "download" with won't be touched at all. The account you "upload to" will receive all the subscriptions and blocks of the downloaded account.
Sounds good. Let me know how it goes. Hopefully it's just instance instability.