Thom won't be cut. People are forgetting that TV and film productions are rarely, if ever, shot in order, from one scene to the next. Say there are 8-10 episodes in season 1. Perhaps they are shooting scenes that don't have Thom in them. As he is only in the first part of EoTW, maybe they're shooting scenes close to the Blight first. Who is in those scenes exclusively? The actors that have been cast/revealed. Except of course Loial. I'd bet they will be heavily relying on CGI for Loial, and heavy practical effects. They may only have a stand-in for him, with an actual actor doing the voice which is done in post.
Thom is essential. He's in it, I have no doubts.