The Lodge Alley Inn on East Bay is awesome. I rented from someone here on TUG at a very reasonable rate (It's BlueGreen). I've also seen ads on TUG for The Church Street Inn. I've never stayed there, but have walked by it and it looked nice (real close to the City Market). I actually picked up reservations for The Mills House on the myclubwyndham site, must have hit it just right (that was in Jan 2016). And even at 50% off I did not use them. It was cheaper to use Wyndham Rewards (and I think that has changed, but not sure). We only stayed a few nights there, we stayed some out at Edisto. When it was just me I stayed at the Days Inn (free parking, lol). I was there for weeks waiting for my grandson to be born (and at the time 'worked from home'). I also stayed at the Andrew Pinckney Inn a few nights. As you know, everything is expensive in Charleston, but I found some kind of deal there for a few nights when my mom and sister were joining me.
My best guess is The Mills House makes 1 room available per week. If that's Wyndham's call or Mills House, I don't know. My conspiracy theory is that by doing so Wyndham can advertise that places like this are available to Wyndham owners. Mills House isn't the only one like this. Because I followed it's availability very closely for a few years, I'm pretty confident that's how things work for The Mills House. It is a beautiful place, if you can figure out a way to stay there.
Looking at The Mills House availability now, my guess is the annual allotment of 1 room per night has been made. Note it's very spotty (1 night here and there) until late in the year. Jan/Feb/March of 2021 - nada. At some point they'll dump rooms in there for 2021, but not til the end of the year, if I remember correctly (I also don't think cancellations ever come back (unless things have changed). And what does 'No product seasons found for the resort(s)' mean? Looking out to November where there are rooms, when I select the checkin date I get that message. No clue.