About the undiagnosed numbers and deadliness compared to flu, I think the Shincheonji of South Korea might be a decent indicator. 5066 cases in SK (more than half of all known SK cases), part of a 230000 member cult that is under close scrunity in SK. (Note link is data from the 24th because I couldn't be bothered to look for the equivalent for yesterday.) They don't say directly how many of the death were among the Shincheonji, but 4383 Shincheonji cases in Daegu alone making it the majority of cases in Daegu, and Daegu CFR matches the rest of the country so it probably isn't that the Shinchonji just have more of their cases known. Anyway with about 1.4% CFR (though SKs CFR has grown a bit more the last days) to get it down to flu deadliness (<0.1% I think?) would require there to be 14*5000=70000 in the cult of 230000, that would be a rather massive testing failure on SK's part. (Also the church isn't just in daegu so 4300 cases there change the numbers further. But I can't find data on how many of the cult are in Daegu.)

SK's data is a huge part of why I doubt the more extreme unknown case numbers/low IFR suggestions. We don't know the true rate yet but I doubt it will be flu level. (Honestly the best numbers will probably in a year or so when we compare this year to other years.)