Limiting estates to FCs wouldn't work because FCs only give a select few housing and gardening permissions. That leaves the rest of the FC members who might want to do those things out in the cold.

All you end up with is players making dummy FCs to get around the FC restriction, exactly as we're seeing right now with the personal moratorium in effect. To make such a restriction effective you'd also have to put conditions on what the FC has to do to retain the housing, not just purchase it.

Apartments/FC member rooms get a lukewarm reception because they're small spaces with a door in the worst possible place that are hard to decorate with housing items that are frequently bulkier than they need to be.

If you really want to lower demand for estates to those who would actually use them, you need to attach a monthly fee to them as opposed to increasing the one time cost. People are unlikely to get something that requires a monthly fee if they aren't really using it.