The specific claims here that are most compelling to me are:
"The grant proposal very specifically is concerned with using the WIV1 and SHC014 backbones, nothing related to SARS-CoV-2." The counterargument is "Well, they could have altered the proposal when they pursued funding elsewhere." The takeaway is that it is not likely this specific proposal was funded elsewhere, leading to SARS-CoV-2.
"If they actually carried out the research in this grant proposal you don't get from there to SARS-CoV-2, those are all SARS-1-like." I do not personally know how to evaluate the accuracy of this claim, but if true, it resonates with the first claim: this proposal funded elsewhere would not lead to SARS-CoV-2.
The argument claims about it being hard/expensive I think are less compelling, as there is a lead time of several years with experts in field performing research. A more compelling version of this argument would look like (completely making up numbers): "On average, it takes 4.5 years to develop the first samples of a novel virus using a selected backbone, CRISPR technology, and gain-of-function culturing. Therefore, even if this research was funded in 2018 we would not expect it to have led to SARS-CoV-2". I'm not saying that argument is accurate at all, just saying it's more specific than "it's difficult".
The argument claims about the evidence being missing I think isn't going to be motivating for a person who has a reasonable expectation that secret research is done and does not have trust in government transparency (either US, China, or otherwise). I'm not making a point here that evidence isn't needed (far from it, evidence IS needed). I'm evaluating from a polemic perspective what kinds of claims and arguments are useful for advancing the conversation with someone who holding a dissonant viewpoint.
Thank you by the way for making specific claims that can be fact checked such as the two referenced at the top of this comment.