It wont let me sign i
If you're using the itch.io PC version, it's no longer supported and you can't use an account on this version of the game. Your options are to use the provided Steam key (it's on your "download" page) or to contacted itch.io for a refund if you can't or won't use Steam (there's a variety of reasons people are avoiding Steam in the past few years). If you're doing the latter, I'd suggest also contacting Innersloth to let them know that you want the game to be supported on here.
No, my problem is the client itself. The only way that currently exists to work around the problem would prevent me from uninstalling games, managing downloads, and accessing most information about the content I'm supposed to own. And even doing that, there's a high risk of a simple misclick causing a migraine which has a risk of putting me in hospital because of how severe I get them. None of which is an acceptable workaround, particularly not 18 months after Valve was first made aware of the problem. There WAS a better workaround to begin with but it was a third-party option not supported by Valve and they've since blocked access to it.