So sideloading directly with Sideloady did work, but that's not really exactly what I had intended to do. As far as I am aware for Sideloady, you can only use developer accounts if you have the actual login for them, correct? I have my device registered on a development team for which I only have access to the certificate and provisioning file. So with Sideloady, I'm limited to the 7-day free developer account.
I was figuring, perhaps naively, that the Cydia Impactor IPA was just like any other IPA from appdb.to and ready to be signed through a tool like iOS App Signer and sideloaded through Xcode. I guess that's not the case.